virtio-blk: remove need for explicit x-data-plane=on option

The x-data-plane=on|off option is no longer useful because the
iothread=<iothread> option conveys the same information plus which
IOThread to use.

Do not delete x-data-plane=on|off yet as a convenience to people using
this legacy experimental option.  We will drop it in QEMU 2.2.

Instead, turn on data-plane when either x-data-plane=on or
iothread=<iothread> are used.  The following command-line uses
data-plane:

  qemu -device virtio-blk-pci,iothread=foo,drive=drive0

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -132,7 +132,7 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *blk,
 
     *dataplane = NULL;
 
-    if (!blk->data_plane) {
+    if (!blk->data_plane && !blk->iothread) {
         return;
     }