rbd: define image specification structure

Group the fields that uniquely specify an rbd image into a new
reference-counted rbd_spec structure.  This structure will be used
to describe the desired image when mapping an image, and when
probing parent images in layered rbd devices.  Replace the set of
fields in the rbd device structure with a pointer to a dynamically
allocated rbd_spec.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>