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.

Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Rusty Russell <[e-mail]>

drivers/block/virtio_blk.c

···
 	}
 
 	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);