x540 and loopback using a dongle

For the x540 card, can somebody point out how to turn loopback on?  What we basically want to do, is to plug in a (loopback) dongle in one of the ports so that the link comes up.   This is for using SR-IOV.  We are running XenServer and a CentOS VM on it.  We create virtual functions and pass one of them to the CentOS VM.  However, for the link to be up on the VF, we need the physical function link to be up.

I believe the lanconf tool has to be used to set the appropriate register in the NIC.  However, I haven't been able to find appropriate documentation of how to do this.