While playing around with the bios.bootDeviceClasses parameter (as shown in this example ), we found out that
- a device not specified in allow: would still be used if all "allow:"ed devices are unusable (no CD connected, no PXE server found, etc.)
- a device specified in deny: would still be used if all other devices are unusable.
So contrary to what the documentation suggests, "allow:" will just move certain devices to the front of the boot device list, and "deny:" moves those devices to the end of the list.
Hope this can help other people trying to make sense of setting boot order in a VM to achieve a specific behavior. In our case: get a VM to reliably boot from CD for automated deployment using the SDK.