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

xen-blkfront: 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.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit ff595325ed556fb4b83af5b9ffd5c427c18405d7)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937