virtio_blk: revert QUEUE_FLAG_VIRT addition

It seems like the addition of QUEUE_FLAG_VIRT caueses major performance regressions for Fedora users:

	https://bugzilla.redhat.com/show_bug.cgi?id=509383
	https://bugzilla.redhat.com/show_bug.cgi?id=505695

while I can't reproduce those extreme regressions myself I think the flag is wrong.

Rationale:

QUEUE_FLAG_VIRT expands to QUEUE_FLAG_NONROT which casus the queue unplugged immediately. This is not a good behaviour for at least qemu and kvm where we do have significant overhead for every I/O operations. Even with all the latested speeups (native AIO, MSI support, zero copy) we can only get native speed for up to 128kb I/O requests we already are down to 66% of native performance for 4kb requests even on my laptop running the Intel X25-M SSD for which the QUEUE_FLAG_NONROT was designed.
If we ever get virtio-blk overhead low enough that this flag makes sense it should only be set based on a feature flag set by the host.

Index: linux-2.6/drivers/block/virtio_blk.c
===================================================================
@@ -314,7 +314,6 @@ static int __devinit virtblk_probe(struc
 	}
 
 	vblk->disk->queue->queuedata = vblk;
-	queue_flag_set_unlocked(QUEUE_FLAG_VIRT, vblk->disk->queue);
 
 	if (index < 26) {
 		sprintf(vblk->disk->disk_name, "vd%c", 'a' + index % 26);

I agree with that rationale.