block/rbd: update s->image_size in qemu_rbd_getlength

While at it just call rbd_get_size and avoid rbd_image_info_t.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Message-Id: <20210702172356.11574-4-idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>