xen-blkfront: set queue paravirt flag

Xen's blkfront sets noop as the default I/O scheduler at initialization
time to avoid elevator overheads such as idling, but with the advent of
basic disk profiling capabilities this is not necessary anymore. We
should just tell the block layer that we are a paravirt front-end driver
and the elevator will automatically make the necessary adjustments.
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/xen-blkfront.c

@@ -338,18 +338,12 @@ wait:
 static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
 {
 	struct request_queue *rq;
-	elevator_t *old_e;
 
 	rq = blk_init_queue(do_blkif_request, &blkif_io_lock);
 	if (rq == NULL)
 		return -1;
 
-	old_e = rq->elevator;
-	if (IS_ERR_VALUE(elevator_init(rq, "noop")))
-		printk(KERN_WARNING
-			"blkfront: Switch elevator failed, use default\n");
-	else
-		elevator_exit(old_e);
+	queue_flag_set_unlocked(QUEUE_FLAG_VIRT, rq);
 
 	/* Hard sector size and max sectors impersonate the equiv. hardware. */
 	blk_queue_hardsect_size(rq, sector_size);