rbd: set up watch before announcing disk

We're ready to handle header object (refresh) events at the point we
call rbd_bus_add_dev().  Set up the watch request on the rbd image
header just after that, and after we've registered the devices for
the snapshots for the initial snapshot context.  Do this before
announce the disk as available for use.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>