Daniel,
navigate to the download site of the firmware for your controller.
Download the CP020320.scexe file; check it's MD5 sum (should be 1dc0880c5526efa382a86f727b788336).
Stop all running VMs: you will need to reboot the server.
Login to your XEN server via ssh. Copy the file to /tmp. Make the file executable. Execute the file (./CP020320.scexe). Follow the instructions.
Cold reboot the server (complete shut down, 10 sec wait, unplug the power cord, all the same backwards).
Have fun...
