virtio/migration: Add VMStateDescription to VirtioDeviceClass

Provide a vmsd pointer for VirtIO devices to use instead of the
load/save methods.

We'll eventually kill off the load/save methods.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -1635,6 +1635,10 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
         vdc->save(vdev, f);
     }
 
+    if (vdc->vmsd) {
+        vmstate_save_state(f, vdc->vmsd, vdev, NULL);
+    }
+
     /* Subsections */
     vmstate_save_state(f, &vmstate_virtio, vdev, NULL);
 }
@@ -1781,6 +1785,13 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
         }
     }
 
+    if (vdc->vmsd) {
+        ret = vmstate_load_state(f, vdc->vmsd, vdev, version_id);
+        if (ret) {
+            return ret;
+        }
+    }
+
     /* Subsections */
     ret = vmstate_load_state(f, &vmstate_virtio, vdev, 1);
     if (ret) {
@@ -2118,6 +2129,9 @@ static void virtio_device_realize(DeviceState *dev, Error **errp)
     VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(dev);
     Error *err = NULL;
 
+    /* Devices should either use vmsd or the load/save methods */
+    assert(!vdc->vmsd || !vdc->load);
+
     if (vdc->realize != NULL) {
         vdc->realize(dev, &err);
         if (err != NULL) {

include/hw/virtio/virtio.h

@@ -125,8 +125,12 @@ typedef struct VirtioDeviceClass {
      * must mask in frontend instead.
      */
     void (*guest_notifier_mask)(VirtIODevice *vdev, int n, bool mask);
+    /* Saving and loading of a device; trying to deprecate save/load
+     * use vmsd for new devices.
+     */
     void (*save)(VirtIODevice *vdev, QEMUFile *f);
     int (*load)(VirtIODevice *vdev, QEMUFile *f, int version_id);
+    const VMStateDescription *vmsd;
 } VirtioDeviceClass;
 
 void virtio_instance_init_common(Object *proxy_obj, void *data,