rbd: don't have device release destroy rbd_dev

Currently an rbd_device structure gets destroyed from the release
routine for the device embedded within it.  Stop doing that, instead
calling rbd_dev_image_release() right after rbd_bus_del_dev()
wherever the latter is called.

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