rbd: Fix rbd diff for non-existent objects

Fixes: #6926
Signed-off-by: Adam Crume <adamcrume@gmail.com>
Please see http://ceph.com/docs/master/man/8/rbd/.

> diff [image-name] [–from-snap snapname]
> Dump a list of byte extents in the image that have changed since the specified start snapshot, or since the image was created. Each output line includes the starting offset (in bytes), the length of the region (in bytes), and either ‘zero’ or ‘data’ to indicate whether the region is known to be zeros or may contain other data.

So `rbd diff img@snap1` should show extents in img@snap1 that have changed since the image was created.  To do what you want, you would run `rbd diff img --from-snap=snap1`.