def private_config_file(self):
        """
        Returns the private-config file for this IOU VM.

        :returns: path to config file. None if the file doesn't exist
        """

        path = os.path.join(self.working_dir, 'private-config.cfg')
        if os.path.exists(path):
            return path
        else:
            return None
Returns the private-config file for this IOU VM.

        :returns: path to config file. None if the file doesn't exist