kvm: Make virtio block device read-only

Signed-off-by: Pekka Enberg <penberg@kernel.org>

blk-virtio.c

@@ -33,6 +33,7 @@ static struct device device = {
 		.blk_size		= 4096,
 	},
 	.host_features		= (1UL << VIRTIO_BLK_F_GEOMETRY)
+				| (1UL << VIRTIO_BLK_F_RO)
 				| (1UL << VIRTIO_BLK_F_BLK_SIZE),
 };