xen, blkback: make request_pending structures per-ring

This commit implements a second step of transition towards per-ring
locking. It moves all the fields of the xen_blkif structure related
to pending_requests to be per-ring.
xen, blkback: move blk_ring_lock to be per-ring and not per-device

This commit implements a further step of transition towards locking
being entirely per-ring and no more per-device. More in detail, this
just moves the lock protecting the access to the ringbuffer to be
per-ring.