You can get the Bios serial number from a hyper-v vm the same way you would get the service tag from a pc using the following command.
wmic bios get serialnumber
You get a number like:
0000-0000-0000-0000-0000-0000-00
This number should remain constant. how ever i do can't confirm this as i have not tested what happens when the VM is moved from hyper-v server to another or even to another vm platform like virtual box.