Call is_outdated() on volume object, not name

Note that this is still a no-op, because admin.vm.volume.Info does not
transmit is_outdated, so the qubesadmin wrapper defaults it to False.

qubesmanager/table_widgets.py

@@ -377,7 +377,7 @@ class VmUpdateInfoWidget(QtGui.QWidget):
         outdated_state = False
 
         try:
-            for vol in vm.volumes:
+            for vol in vm.volumes.values():
                 if vol.is_outdated():
                     outdated_state = "outdated"
                     break