frontend: add syntax for inheritance
fix inheritance fallout

Several places need updating to handle the new virtual classes.
openflow_input: add inheritance
locitest: dont test virtual classes
locitest: dont validate virtual messages
Merge into master from pull request #41:
add explicit inheritance to loxi input files (https://github.com/floodlight/loxigen/pull/41)
Merge remote-tracking branch 'rlane/explicit-inheritance2'

Conflicts:
	loxi_front_end/frontend.py
	loxi_front_end/parser.py
	loxi_ir.py
frontend: add support for discrimiator fields

In an abstract base class (e.g., of_action), the dicriminator field
represents the field that distinguishes between different type of
subclasses. It is marked in the base class with a '== ?', as in

   uint32_t type == ?

Each class may only define at most one discriminator field.
openflow_input: add discriminator fields
Merge branch 'master' of github.com:floodlight/loxigen