Fix up python bindings for new event callbacks

Fix up python bindings for new event callbacks

The generator was disabled for the new event callbacks, since they
need to be hand written. This patch  adds the C and python glue to
expose the new APIs in the python binding. The python example
program is extended to demonstrate of the code

* python/libvirt-override.c: Registration and dispatch of events
   at the C layer
* python/libvirt-override-virConnect.py: Python glue for events
* examples/domain-events/events-python/event-test.py: Demo use
  of new event callbacks