pyloxi: move version-independent utility code to a new file

Also adds a test for unpack_list.
diff --git a/py_gen/templates/message.py b/py_gen/templates/message.py
index 8eb051d..d501854 100644
--- a/py_gen/templates/message.py
+++ b/py_gen/templates/message.py
@@ -37,6 +37,7 @@
 import common
 import action # for unpack_list
 import util
+import loxi.generic_util
 
 class Message(object):
     version = const.OFP_VERSION