MAC addresses on virtual installations such as VirtualBox are not connected to hardware.
Do they have any absolute meaning, and are they registered somewhere, the way hardware MAC addresses are supposed to be? Or are they essentially spoofed?
Essentially they are spoofed.
They can indicate that the unit is a VM but they don't have to. They are generating a MAC address for a virtual adapter so they don't have the opportunity to be "real". How they are generated all depends on the Virtual Machine manufacturer. Here are a few links to how different vendors implement MAC addresses: