virtio: recalculate vq->inuse after migration

The vq->inuse field is not migrated.  Many devices don't hold
VirtQueueElements across migration so it doesn't matter that vq->inuse
starts at 0 on the destination QEMU.

At least virtio-serial, virtio-blk, and virtio-balloon migrate while
holding VirtQueueElements.  For these devices we need to recalculate
vq->inuse upon load so the value is correct.

Cc: qemu-stable@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -1648,6 +1648,21 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
             }
             vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
             vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);
+
+            /*
+             * Some devices migrate VirtQueueElements that have been popped
+             * from the avail ring but not yet returned to the used ring.
+             */
+            vdev->vq[i].inuse = vdev->vq[i].last_avail_idx -
+                                vdev->vq[i].used_idx;
+            if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {
+                error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "
+                             "used_idx 0x%x",
+                             i, vdev->vq[i].vring.num,
+                             vdev->vq[i].last_avail_idx,
+                             vdev->vq[i].used_idx);
+                return -1;
+            }
         }
     }