The Windows 10 auto-activation system is quite nice: you no longer need to type in serial numbers to re-install the OS on the same machine - the machine's identity is used to activate it.
When doing a free upgrade you don't even get a Windows 10 serial number - your activation is simply linked to the machine's identity that you are upgrading.
How does this work on a virtual machine (not specifically through Microsoft's special VM server with its own activation system, but any, like VirtualBox)? These machines can be made identical everywhere. I guess VirtualBox (and presumably others) create a random MAC address for each new machine, but they also give you the ability to change it easily. Everything else is likely to be nearly identical in different VMs. Could someone just share the MAC address and get a Windows 10 VM that is activated?
Or maybe, since Windows knows it is a VM, will it require a license key? In this case, will a free upgrade not work?
[I will be updating my Windows 8.1 VirtualBox VM to Windows 10 and wondering how this will work]