SEBA-466 Fix or note potential concurrency issues

Change-Id: Iabfa6c7021cacc565c39e09fcd884fa048ca5004
diff --git a/xos/synchronizer/model_policies/model_policy_voltserviceinstance.py b/xos/synchronizer/model_policies/model_policy_voltserviceinstance.py
index 966435a..c437c78 100644
--- a/xos/synchronizer/model_policies/model_policy_voltserviceinstance.py
+++ b/xos/synchronizer/model_policies/model_policy_voltserviceinstance.py
@@ -68,4 +68,4 @@
             raise Exception("ONUDevice with serial number %s can't be found" % onu_device_serial_number)
 
         si.onu_device_id = onu.id
-        si.save()
+        si.save_changed_fields()