rbd: fill rbd_spec in rbd_add_parse_args()

Pass the address of an rbd_spec structure to rbd_add_parse_args().
Use it to hold the information defining the rbd image to be mapped
in an rbd_add() call.

Use the result in the caller to initialize the rbd_dev->id field.

This means rbd_dev is no longer needed in rbd_add_parse_args(),
so get rid of it.

Now that this transformation of rbd_add_parse_args() is complete,
correct and expand on the its header documentation to reflect the
new reality.

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