the qemu rbd driver currently lacks support for bdrv_co_block_status.
This results mainly in incorrect progress during block operations (e.g.
qemu-img convert with an rbd image as source).

This patch utilizes the rbd_diff_iterate2 call from librbd to detect
allocated and unallocated (all zero areas).

To avoid querying the ceph OSDs for the answer this is only done if
the image has the fast-diff feature which depends on the object-map and
exclusive-lock features. In this case it is guaranteed that the information
is present in memory in the librbd client and thus very fast.

If fast-diff is not available all areas are reported to be allocated
which is the current behaviour if bdrv_co_block_status is not implemented.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <20211012152231.24868-1-pl@kamp.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>