mfi-*: Set appropriate PropMinVer:XenMin: hostflags, to honour XenMin property

mfi-*: Set appropriate PropMinVer:XenMin: hostflags, to honour XenMin property

* In test_matrix_iterate, where most_hostflags is set, include a
  PropMinVer:XenMin: hostflag.  This is derived from $xenbranch.
  When $xenbranch is xen-unstable, don't add that hostflag.

* But this is wrong for the migrate upgrade tests, which use both this
  and the previous version of Xen.  It is sufficient to check the
  previous version, so we strip out the existing PropMinVer if there
  is one.  We (ab)use select_prevxenbranch to calculate the old Xen
  branch name (version).

The motivation right now for this is that Xen 4.10 and earlier do not
boot xen.gz on UEFI.  In earlier versions, one has to chainload
xen.efi.  We don't support that in osstest right now on x86 (and it
probably isn't worth fixing that logic).

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>