xen-blkfront: fix mq start/stop race

When ring buf full, hw queue will be stopped. While blkif interrupt consume
request and make free space in ring buf, hw queue will be started again.
But since start queue is protected by spin lock while stop not, that will
cause a race.

interrupt:                                      process:
blkif_interrupt()                               blkif_queue_rq()
 kick_pending_request_queues_locked()
   blk_mq_start_stopped_hw_queues()
      clear_bit(BLK_MQ_S_STOPPED, &hctx->state)
	                                             blk_mq_stop_hw_queue(hctx)
	  blk_mq_run_hw_queue(hctx, async)

If ring buf is made empty in this case, interrupt will never come, then the
hw queue will be stopped forever, all processes waiting for the pending io
in the queue will hung.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Ankur Arora <ankur.a.arora@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>