Fix for PONSIM exit issue with execption

Change-Id: Id249cb67760c8aa5708dccca63b60f06608d4900
diff --git a/ponsim/main.py b/ponsim/main.py
index f3d0ffd..3e32d3a 100755
--- a/ponsim/main.py
+++ b/ponsim/main.py
@@ -174,11 +174,11 @@
         """Execute before the reactor is shut down"""
         self.log.info('exiting-on-keyboard-interrupt')
         if self.io is not None:
-            self.io.stop()
+            yield self.io.stop()
         self.teardown_networking_assets(self.args.name, self.args.onus)
         if self.grpc_server is not None:
             yield self.grpc_server.stop()
-
+        
     def start_reactor(self):
         from twisted.internet import reactor
         reactor.callWhenRunning(