xen/blkback: Get the 'requeust_queue' properly.

After the commit 0faa8cca
("    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>