virtio: Use started flag in virtio_vmstate_change()

Currently, we use DRIVER_OK status bit to check whether guest
driver has started the device in virtio_vmstate_change(). But it's
not the case for virtio 1.0 transitional devices. If migration completes
between kicking virtqueue and setting VIRTIO_CONFIG_S_DRIVER_OK, guest
may be hung. So here we use started flag to check guest state instead.
Signed-off-by: NXie Yongji <xieyongji@baidu.com>
Signed-off-by: NZhang Yu <zhangyu31@baidu.com>
Message-Id: <20190320112646.3712-3-xieyongji@baidu.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -2291,7 +2291,7 @@ static void virtio_vmstate_change(void *opaque, int running, RunState state)
     VirtIODevice *vdev = opaque;
     BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
-    bool backend_run = running && (vdev->status & VIRTIO_CONFIG_S_DRIVER_OK);
+    bool backend_run = running && vdev->started;
     vdev->vm_running = running;
 
     if (backend_run) {