I've had a virtual machine suspended on VirtualBox v4.2.16. Carelessly, I upgraded to v4.3.20, without powering down all VM clients...
So, when I tried to resume a VM after the update of VirtualBox, I got the following error message:
[SOLUTION]Failed to load unit 'cpum'
I was afraid that I had lost that VM-client - and I still couldn't figure out how to resume the machine.
Actually, I haven't found a solution, but a reasonable workaround:
I've told VirtualBox to discard the saved machine state, and then restarted it.