qemu: migration: Improve handling of VIR_MIGRATE_PARAM_DEST_XML with VIR_MIGRATE_PERS...

qemu: migration: Improve handling of VIR_MIGRATE_PARAM_DEST_XML with VIR_MIGRATE_PERSIST_DEST

When a user provides a migration XML via the VIR_MIGRATE_PARAM_DEST_XML
it's expected that they want to change ABI-compatible aspects of the XML
such as the disk paths or similar.

If the user requests persisting of the VM but does not provide an
explicit persistent XML libvirt would take the persistent XML from the
source of the migration as the persistent config. This usually involves
the old paths to images.

Doing this would result into failure to start the VM.

It makes more sense to take the XML used for migration and use that as
the base for persisting the config.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>