qemu_snapshot: fix reverting to inactive snapshot

When reverting to inactive snapshot updating the domain definition needs
to happen after the new overlays are created otherwise qemu-img will
correctly fail with error:

    Trying to create an image with the same filename as the backing file

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>