xen-blkback: constify instance of "struct attribute_group"

The functions these get passed to have been taking pointers to const
since at least 2.6.16.

Acked-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>