pyloxi: create inheritance hierarchy

Virtual classes are generated but currently only serve as superclasses.

Somehow this works without topologically sorting the classes.
pyloxi: generate virtual class unpack methods

This is done completely from the IR now.
pyloxi: move list unpack functions into util

The goal is to make all module templates identical.
pyloxi: generate all openflow class modules from a single template

common.py and message.py had small amounts of special code left which were
moved into the *_extra.py subtemplates.
pyloxi: remove automatic conversion from str to OFReader

User code shouldn't call these methods directly.