Returns the startup script file for this VPCS VM.

        :returns: path to startup script file

def script_file(self):
        """
        Returns the startup script file for this VPCS VM.

        :returns: path to startup script file
        """

        # use the default VPCS file if it exists
        path = os.path.join(self.working_dir, 'startup.vpc')
        if os.path.exists(path):
            return path
        else:
            return None