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

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.
(cherry picked from commit 2fb1ef4f1226ea6d6d3481036cabe01a4415b68c)
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>