throttle: test that snapshots move the throttling configuration

If a snapshot is performed on a device that has I/O limits they should
be moved to the target image (the new active layer).

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
