block/backup: add backup_is_cluster_allocated

block/backup: add backup_is_cluster_allocated

Modify bdrv_is_unallocated_range to utilize the pnum return from
bdrv_is_allocated, and in the process change the semantics from
"is unallocated" to "is allocated."

Optionally returns a full number of clusters that share the same
allocation status.

This will be used to carefully toggle bits in the bitmap for sync=top
initialization in the following commits.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20190716000117[email protected]
Signed-off-by: John Snow <[email protected]>