virtio: Update migration docs

Remove references to register_savevm.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

docs/virtio-migration.txt

@@ -28,7 +28,8 @@ virtio core               virtio transport          virtio device
 -----------               ----------------          -------------
 
                                                     save() function registered
-                                                    via register_savevm()
+                                                    via VMState wrapper on
+                                                    device class
 virtio_save()                                       <----------
              ------>      save_config()
                           - save proxy device
@@ -63,7 +64,8 @@ virtio core               virtio transport          virtio device
 -----------               ----------------          -------------
 
                                                     load() function registered
-                                                    via register_savevm()
+                                                    via VMState wrapper on
+                                                    device class
 virtio_load()                                       <----------
              ------>      load_config()
                           - load proxy device