rbd-wnbd: adjust admin socket hook to accept image path

rbd-wnbd: adjust admin socket hook to accept image path

For each rbd-wnbd mapping we set an admin socket hook that can
be used to retrieve IO stats.

Now that the same daemon is reused for multiple mappings, we need
to distinguish the images when receiving a "stats" request.

For this reason, we'll add the image identifier to "wnbd stats"
admin socket commands.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>