pyloxi: use OFClass directly

It now includes everything we need except pyname.
diff --git a/py_gen/templates/oxm.py b/py_gen/templates/oxm.py
index 3d888a7..176a74c 100644
--- a/py_gen/templates/oxm.py
+++ b/py_gen/templates/oxm.py
@@ -58,7 +58,7 @@
 :: #endfor
 
 parsers = {
-:: key = lambda x: x.type_members[0].value
+:: key = lambda x: x.member_by_name('type_len').value
 :: for ofclass in sorted(ofclasses, key=key):
     ${key(ofclass)} : ${ofclass.pyname}.unpack,
 :: #endfor