pyloxi: support OXM member in set-field actions

The C backend needs more work to support of_oxm_t, so I've added a hack so it
can keep using of_octets_t.

Also added several set-field action testcases.
pyloxi: factor out loxi.generic_util.pad_to
pyloxi: slightly generalize alignment

The _pack and _unpack templates should not be matching on particular class
names. Move the decision up a few levels to codegen.
Merge into master from pull request #36:
implement OF 1.3 set-field actions in pyloxi (https://github.com/floodlight/loxigen/pull/36)