rbd: wire up is_visible() sysfs callback for rbd bus

In preparation for single-major device number allocation scheme, wire
up attribute_group::is_visible() callback for rbd bus.  This allows us
to make the new single-major attributes conditional.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>