qemu: Don't update CPU when checking ABI stability

qemu: Don't update CPU when checking ABI stability

When checking ABI stability between two domain definitions, we first
make migratable copies of them. However, we also asked for the guest CPU
to be updated, even though the updated CPU is supposed to be already
included in the original definitions. Moreover, if we do this on the
destination host during migration, we're potentially updating the
definition with according to an incompatible host CPU.

While updating the CPU when checking ABI stability doesn't make any
sense, it actually just worked because updating the CPU doesn't do
anything for custom CPUs (only host-model CPUs are affected) and we
updated both definitions in the same way.

Less then a year ago commit v2.3.0-rc1~42 stopped updating the CPU in
the definition we got internally and only the user supplied definition
was updated. However, the same commit started updating host-model CPUs
to custom CPUs which are not affected by the request to update the CPU.
So it still seemed to work right, unless a user upgraded libvirt 2.2.0
to a newer version while there were some domains with host-model CPUs
running on the host. Such domains couldn't be migrated with a user
supplied XML since libvirt would complain:

    Target CPU mode custom does not match source host-model

The fix is pretty straightforward, we just need to stop updating the CPU
when checking ABI stability.

https://bugzilla.redhat.com/show_bug.cgi?id=1463957

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>