pyloxi: parse unknown subclasses as the superclass

Previously we would throw an exception if we encountered a (for example)
experimenter action subclass that we didn't know existed. This was fine for
OFTest but causes problems for other tools that use pyloxi.

This change generates a full set of pack/unpack/pretty_print functions for
virtual classes. If a discriminator field contains a subtype we don't know
about, the virtual class is instantiated instead.

This also removes the special cases around unknown hello elements.
Merge into master from pull request #186:
pyloxi: parse unknown subclasses as the superclass (https://github.com/floodlight/loxigen/pull/186)