qemu: Use "migratable" XML definition when doing external checkpoints

qemu: Use "migratable" XML definition when doing external checkpoints

In the original implementation of external checkpoints I've mistakenly
used the live definition to be stored in the save image. The normal
approach is to use the "migratable" definition. This was discovered when
commit 07966f6a8b5ccb5bb4c716b25deb8ba2e572cc67 changed the behavior to
use a converted XML from the user to do the compatibility check to fix
problem when using the regular machine saving.

As the previous patch added a compatibility layer, we can now change the
type of the XML in the image.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1008340