qemu: Properly rename persistent def after migration

qemu: Properly rename persistent def after migration

When migrating a domain while changing its name and using
VIR_MIGRATE_PERSIST_DEST flag, libvirt would fail to properly change the
name in the persistent definition. The inconsistency results in weird
behavior when dumping domain XML, destroying the domain, restarting
libvirtd and likely in several other situations.

Since the new name is already stored in vm->def->name, we just need to
make sure the persistent definition uses this new name too.

https://bugzilla.redhat.com/show_bug.cgi?id=1076354

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>