<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _check_vbox_port_forwarding(self):
 """ Checks if the NAT port forwarding rule exists. :returns: boolean """


        result = yield from self._execute("showvminfo", [self._vmname, "--machinereadable"])
        for info in result.splitlines():
            if '=' in info:
                name, value = info.split('=', 1)
                if name.startswith("Forwarding") and value.strip('"').startswith("GNS3VM"):
                    return True
        return False