xen/blkback: Remove unnecessary static variable name prefixes

A few of static variables in blkback have 'xen_blkif_' prefix, though it
is unnecessary for static variables.  This commit removes such prefixes.

Reviewed-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>