xen-blkback: prefer xenbus_scanf() over xenbus_gather()

xen-blkback: prefer xenbus_scanf() over xenbus_gather()

... for single items being collected: It is more typesafe (as the
compiler can check format string and to-be-written-to variable match)
and requires one less parameter to be passed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 6694389af9be4d1eb8d3313788a902f0590fb8c2)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937