11.40am. Well that was easy. I upgraded our virtualisation infrastructure to Xenserver 5.6 a couple of weeks ago but at the time I forgot to upgrade the XenTools packages. With 5 minutes spare today I was able to complete this task by performing the following:

* Attach the XenTools ISO by selecting VM->Install XenServer Tools.
* Mount the DVD:
mount /dev/xvdd /mnt
* Run the script on the mounted DVD.
./mnt/Linux/install.sh
* Unmount the DVD
umount /mnt
* Restart the VM.
* Check under Virtualisation State (found under the general tab for that VM) it says: Optimized

Fin.