After the commit 0faa8cca883bbc6a0919e3c89128672659b75820
(" xen/blkback: remove per-queue plugging") we forgot
to retrieve the 'struct request_queue' from the block device.
This puts the functionality back in and fixes a NULL pointer
bug.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>