We need to release any block migrations BlockBackends on the source
before successfully completing the migration because otherwise
inactivating the images will fail (inactivation only tolerates device
BBs).
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>