loxi-prep: remove uses of cstruct module

This module does not exist in PyLoxi.
diff --git a/tests/port_stats.py b/tests/port_stats.py
index 5e0648f..d2ade19 100644
--- a/tests/port_stats.py
+++ b/tests/port_stats.py
@@ -11,7 +11,7 @@
 
 from oftest import config
 import oftest.controller as controller
-import oftest.cstruct as ofp
+import ofp
 import oftest.message as message
 import oftest.dataplane as dataplane
 import oftest.action as action