Internal method.  Creates threads to handle low-level
        network receive.

def _spawn_threads(self):
        '''
        Internal method.  Creates threads to handle low-level
        network receive.
        '''
        for devname,pdev in self._pcaps.items():
            t = threading.Thread(target=LLNetReal._low_level_dispatch, args=(pdev, devname, self._pktqueue))
            t.start()
            self._threads.append(t)