pyloxi: move OFType interface functions to module level

This removes the need to replace the LOXI IR OFType object in members with our
own version. If/when the core OFType is upgraded to an inheritance hierachy we
won't need to duplicate it in pyloxi.
diff --git a/py_gen/codegen.py b/py_gen/codegen.py
index fb2e48c..4e93c4c 100644
--- a/py_gen/codegen.py
+++ b/py_gen/codegen.py
@@ -67,20 +67,12 @@
 
         for m in ofclass.members:
             if type(m) == OFTypeMember:
-                members.append(OFTypeMember(
-                    name=m.name,
-                    oftype=oftype.OFType(m.oftype, version),
-                    value=m.value))
+                members.append(m)
                 type_members.append(members[-1])
             elif type(m) == OFLengthMember:
-                members.append(OFLengthMember(
-                    name=m.name,
-                    oftype=oftype.OFType(m.oftype, version)))
+                members.append(m)
             elif type(m) == OFFieldLengthMember:
-                members.append(OFFieldLengthMember(
-                    name=m.name,
-                    oftype=oftype.OFType(m.oftype, version),
-                    field_name=m.field_name))
+                members.append(m)
             elif type(m) == OFPadMember:
                 members.append(m)
             elif type(m) == OFDataMember:
@@ -88,13 +80,11 @@
                     # HACK move to frontend
                     members.append(OFTypeMember(
                         name=m.name,
-                        oftype=oftype.OFType(m.oftype, version),
+                        oftype=m.oftype,
                         value=version))
                     type_members.append(members[-1])
                 else:
-                    members.append(OFDataMember(
-                        name=m.name,
-                        oftype=oftype.OFType(m.oftype, version)))
+                    members.append(m)
 
         ofclasses.append(
             PyOFClass(name=cls,