xen/blkback: Consistently insert one empty line between functions

The number of empty lines between functions in the xenbus.c is
inconsistent.  This trivial style cleanup commit fixes the file to
consistently place only one empty line.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>