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>
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v2: Align variables in the structures.