Comment 2 for bug 1785859

I ran qemu (in cosmic) with a cloud image and

 -device virtio-net-pci,netdev=mynet,mac=$mymac -netdev user,id=mynet

and loaded the ofnet module. I then listed the network cards and the mac address was correctly recognized.

I have not figured out how to PXE boot it yet, so net_default_mac was empty.

But this experience leads me to believe that there are multiple interfaces and it might be looking at the wrong one. There does not seem to be any code in grub to generate random mac addresses.