Convert input XML to migratable before using it in
qemuDomainSaveImageOpen.

XML in the save image is migratable, i.e. doesn't contain implicit
controllers. If these controllers were in a non-default order in the
input XML, the ABI check would fail. Removing and re-adding these
controllers fixes it.

https://bugzilla.redhat.com/show_bug.cgi?id=834196
(cherry picked from commit 07966f6a)