libvirt: disable Werror for non-libvirt flights

libvirt: disable Werror for non-libvirt flights

Current usage of Werror=switch-enum by default for libvirt builds out
of the git tree causes issues when new items are added to libxl public
API enums if those are used in a switch statement in libvirt code.
This leads to libvirt build failures for seemingly unrelated libxl
changes.

In order to prevent those errors from blocking the push gate, disable
Werror for libvirt builds when not in a libvirt specific flight.

The errors will be reported on the libvirt flight, and block the
pushes there.  So the author of the changes in libxl is still expected
to send a fix to libvirt code.  This is no ideal, but the other option
is to just disable Werror for all libvirt builds and let libvirt
developers fix the breakage when they notice it.

runvar differences for a xen-unstable flight are:

--- /dev/fd/63  2022-09-15 15:57:44.340581730 +0000
+++ /dev/fd/62  2022-09-15 15:57:44.340581730 +0000
@@ -574,6 +574,10 @@
 test-xtf-amd64-amd64-3                                arch                            amd64
 test-xtf-amd64-amd64-4                                arch                            amd64
 test-xtf-amd64-amd64-5                                arch                            amd64
+build-amd64-libvirt                                   autogen_options                 --disable-werror
+build-arm64-libvirt                                   autogen_options                 --disable-werror
+build-armhf-libvirt                                   autogen_options                 --disable-werror
+build-i386-libvirt                                    autogen_options                 --disable-werror
 test-amd64-amd64-libvirt-qemuu-debianhvm-amd64-xsm    bios                            seabios
 test-amd64-amd64-qemuu-nested-amd                     bios                            seabios
 test-amd64-amd64-qemuu-nested-intel                   bios                            seabios
@@ -1217,6 +1221,10 @@
 build-arm64-libvirt                                   make_njobs                      1
 build-armhf-libvirt                                   make_njobs                      1
 build-i386-libvirt                                    make_njobs                      1
+build-amd64-libvirt                                   meson_options                   -Dwerror=false
+build-arm64-libvirt                                   meson_options                   -Dwerror=false
+build-armhf-libvirt                                   meson_options                   -Dwerror=false
+build-i386-libvirt                                    meson_options                   -Dwerror=false
 test-amd64-amd64-xl-qemuu-dmrestrict-amd64-dmrestrict recipe_dmrestrict               true
 test-amd64-i386-xl-qemuu-dmrestrict-amd64-dmrestrict  recipe_dmrestrict               true
 test-amd64-amd64-xl-qemuu-dmrestrict-amd64-dmrestrict recipe_nomigrate                true

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
Changes since v1:
 - Use -Dwerror=false instead of -Dgit_werror=disabled
---
Cc: Ian Jackson <iwj@xenproject.org>
Cc: Anthony PERARD <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>