xen/blkback: pseudo support for multi hardware queues/rings

Preparatory patch for multiple hardware queues (rings). The number of
rings is unconditionally set to 1, larger number will be enabled in
"xen/blkback: get the number of hardware queues/rings from blkfront".
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
v2: Align variables in the structures.

drivers/block/xen-blkback/common.h

@@ -340,7 +340,8 @@ struct xen_blkif {
 	struct work_struct	free_work;
 	unsigned int 		nr_ring_pages;
 	/* All rings for this device. */
-	struct xen_blkif_ring 	ring;
+	struct xen_blkif_ring	*rings;
+	unsigned int		nr_rings;
 };
 
 struct seg_buf {