block: add queue flag for paravirt frontend drivers

As is the case with SSD devices, we do not want to idle in AS/CFQ when
the block device is a paravirt front-end driver. This patch adds a flag
(QUEUE_FLAG_VIRT) which should be used by front-end drivers such as
virtio_blk and xen-blkfront to indicate a paravirtualized device.
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/blkdev.h

@@ -449,6 +449,7 @@ struct request_queue
 #define QUEUE_FLAG_FAIL_IO     12	/* fake timeout */
 #define QUEUE_FLAG_STACKABLE   13	/* supports request stacking */
 #define QUEUE_FLAG_NONROT      14	/* non-rotational device (SSD) */
+#define QUEUE_FLAG_VIRT        QUEUE_FLAG_NONROT /* paravirt device */
 
 static inline int queue_is_locked(struct request_queue *q)
 {