qemuDomainSaveImageUpdateDef: Don't overwrite errors from virDomainDefCheckABIStability

qemuDomainSaveImageUpdateDef: Don't overwrite errors from virDomainDefCheckABIStability

https://bugzilla.redhat.com/show_bug.cgi?id=1379200

When we are restoring a domain from a saved image, or just
updating its XML in the saved image - we have to make sure that
the ABI guests sees will not change. We have a function for that
which reports errors. But for some reason if this function fails,
we call it again with slightly different argument. Therefore it
might happen that we overwrite the original error and leave user
with less helpful one.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>