loci: replace of_list_iter_t with of_object_t *

All known callers have been switched over.
loci: remove inheritance unions
loci: generate typedefs for inheritance roots
loci: dump/show correct subclass for embedded objects

The code was previously calling the dump/show function for the header class,
which wouldn't have output any of the fields added by the child classes.
loci: remove header classes

Now that e.g. of_oxm_t is a valid typedef for of_object_t, there's no need for
of_oxm_header_t.
loci: include inheritance roots that have no subclasses

of_table_mod_prop needs to be treated like an inheritance root even though
there are no subclasses currently defined.
loci: generate less code and tests for virtual classes

Most uses of type_maps.class_is_inheritance_root are bugs. Generally you
actually want to know if a class is virtual. This is complicated somewhat by
lists also being virtual.
Merge into master from pull request #348:
loci: remove inheritance unions and header classes (https://github.com/floodlight/loxigen/pull/348)