Implements in-order handling for vhost devices using shadow virtqueues.

Since vhost's shadow virtqueues utilize batching in their
vhost_svq_flush calls, the vhost device is responsible for calling
virtqueue_flush once it has completed its batching operation.

Note:
-----
It's unclear if this implementation is really necessary to "guarantee"
in-order handling since, by design, the vhost_svq_flush function puts
used VirtQueueElements in-order already.

Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index fc5f408f77..3c42adee87 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -493,11 +493,20 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
                 qemu_log_mask(LOG_GUEST_ERROR,
                          "More than %u used buffers obtained in a %u size SVQ",
                          i, svq->vring.num);
-                virtqueue_fill(vq, elem, len, i);
-                virtqueue_flush(vq, i);
+                if (virtio_vdev_has_feature(svq->vdev, VIRTIO_F_IN_ORDER)) {
+                    virtqueue_order_element(vq, elem, len, i, i);
+                } else {
+                    virtqueue_fill(vq, elem, len, i);
+                    virtqueue_flush(vq, i);
+                }
                 return;
             }
-            virtqueue_fill(vq, elem, len, i++);
+
+            if (virtio_vdev_has_feature(svq->vdev, VIRTIO_F_IN_ORDER)) {
+                virtqueue_order_element(vq, elem, len, i++, 0);
+            } else {
+                virtqueue_fill(vq, elem, len, i++);
+            }
         }
 
         virtqueue_flush(vq, i);
-- 
2.39.3