<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def startup_config_file(self):
 """ Returns the startup-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, 'startup-config.cfg')
        if os.path.exists(path):
            return path
        else:
            return None