xen, blkback: add ring index to xen_blkif_rings

This commit adds an extra variable to xen_blkif_rings which
represents the ring's index in the array.
xen, blkback: change ring to rings

This commit fixes the ambiguous name of the "ring" field of the
xen_blkif structure. The field keeps the pointer to an array of
xen_blkif_ring structures, so "rings" is more appropriate.
xen, blkback: change XenStore keys to support multiqueue

This commit changes XenStore keys used by Xen's PV block I/O driver
to support the presence of multiple rings. This commit keeps things
simple, still forcing the number of submit queues to 1 and reading
keys as if the driver only had to support multi-queue-capable guests
and kernels.