block/rbd: fix handling of holes in .bdrv_co_block_status

the assumption that we can't hit a hole if we do not diff against a snapshot was wrong.

We can see a hole in an image if we diff against base if there exists an older snapshot
of the image and we have discarded blocks in the image where the snapshot has data.

Fix this by simply handling a hole like an unallocated area. There are no callbacks
for unallocated areas so just bail out if we hit a hole.

Fixes: 0347a8fd
Suggested-by: Ilya Dryomov <idryomov@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <20220113144426.4036493-2-pl@kamp.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>