Arrange to test migration from the previous Xen version

Arrange to test migration from the previous Xen version

There are several steps to this:

- Identify $prevxenbranch, that is the branch which precedes
  $xenbranch.
- Create appropriate build jobs.
- Add support in ts-xen-install for overriding {xen,}buildjob on a
  per-ident basis
- Add a new receipt test-pair-oneway which only migrates from
  src_host to dst_host and not the reverse
- Create appropriate test jobs, overridding the default builds for
  src_host.

Currently we only do this for xen* branches and using xl, but in the
future we may wish to add to the libvirt branch too.

In make-flight if REVISION_PREVXEN is not supplied (e.g. called from
standalone-reset or by hand etc) then we create the build-$arch-prev jobs
with no revision_xen, same as build-$arch

It would be nice to try and reuse the builds from the last flight
which tested the $prevxenbranch baseline. I've not dont that here.

+build-amd64-prev                                      arch                        amd64
+build-amd64-prev                                      build_lvextend_max          50
+build-amd64-prev                                      enable_ovmf                 true
+build-amd64-prev                                      enable_xend                 false
+build-amd64-prev                                      enable_xsm                  false
+build-amd64-prev                                      host_hostflags              share-build-wheezy-amd64,arch-amd64,suite-wheezy,purpose-build
+build-amd64-prev                                      revision_ovmf
+build-amd64-prev                                      revision_qemu
+build-amd64-prev                                      revision_qemuu              c4a962ec0c61aa9b860a3635c8424472e6c2cc2c
+build-amd64-prev                                      revision_seabios
+build-amd64-prev                                      revision_xen                666b80f239c566283cb1b3435180d99a329d0156
+build-amd64-prev                                      tree_ovmf
+build-amd64-prev                                      tree_qemu                   git://xenbits.xen.org/staging/qemu-xen-unstable.git
+build-amd64-prev                                      tree_qemuu                  git://xenbits.xen.org/staging/qemu-upstream-unstable.git
+build-amd64-prev                                      tree_seabios
+build-amd64-prev                                      tree_xen                    git://xenbits.xen.org/xen.git

+build-i386-prev                                       arch                        i386
+build-i386-prev                                       build_lvextend_max          50
+build-i386-prev                                       enable_ovmf                 true
+build-i386-prev                                       enable_xend                 false
+build-i386-prev                                       enable_xsm                  false
+build-i386-prev                                       host_hostflags              share-build-wheezy-i386,arch-i386,suite-wheezy,purpose-build
+build-i386-prev                                       revision_ovmf
+build-i386-prev                                       revision_qemu
+build-i386-prev                                       revision_qemuu              c4a962ec0c61aa9b860a3635c8424472e6c2cc2c
+build-i386-prev                                       revision_seabios
+build-i386-prev                                       revision_xen                666b80f239c566283cb1b3435180d99a329d0156
+build-i386-prev                                       tree_ovmf
+build-i386-prev                                       tree_qemu                   git://xenbits.xen.org/staging/qemu-xen-unstable.git
+build-i386-prev                                       tree_qemuu                  git://xenbits.xen.org/staging/qemu-upstream-unstable.git
+build-i386-prev                                       tree_seabios
+build-i386-prev                                       tree_xen                    git://xenbits.xen.org/xen.git

+test-amd64-amd64-upgrade                              all_hostflags               arch-amd64,arch-xen-amd64,suite-wheezy,purpose-test,equiv-1
+test-amd64-amd64-upgrade                              arch                        amd64
+test-amd64-amd64-upgrade                              buildjob                    build-amd64
+test-amd64-amd64-upgrade                              debian_arch                 amd64
+test-amd64-amd64-upgrade                              debian_kernkind             pvops
+test-amd64-amd64-upgrade                              kernbuildjob                build-amd64-pvops
+test-amd64-amd64-upgrade                              kernkind                    pvops
+test-amd64-amd64-upgrade                              src_host_buildjob           build-amd64-prev
+test-amd64-amd64-upgrade                              src_host_xenbuildjob        build-amd64-prev
+test-amd64-amd64-upgrade                              toolstack                   xl
+test-amd64-amd64-upgrade                              xenbuildjob                 build-amd64

+test-amd64-i386-upgrade                               all_hostflags               arch-i386,arch-xen-amd64,suite-wheezy,purpose-test,equiv-1
+test-amd64-i386-upgrade                               arch                        i386
+test-amd64-i386-upgrade                               buildjob                    build-i386
+test-amd64-i386-upgrade                               debian_arch                 i386
+test-amd64-i386-upgrade                               debian_kernkind             pvops
+test-amd64-i386-upgrade                               kernbuildjob                build-i386-pvops
+test-amd64-i386-upgrade                               kernkind                    pvops
+test-amd64-i386-upgrade                               src_host_buildjob           build-i386-prev
+test-amd64-i386-upgrade                               src_host_xenbuildjob        build-amd64-prev
+test-amd64-i386-upgrade                               toolstack                   xl
+test-amd64-i386-upgrade                               xenbuildjob                 build-amd64

NB the regular build jobs are, as expected, unchanged (and different to the
ones above):

 build-amd64                                           revision_xen                9256f66c1606cd9339412bff7fbc7bd9e8beb28c
 build-i386                                            revision_xen                9256f66c1606cd9339412bff7fbc7bd9e8beb28c

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>