I have a software product. it will probably run on VMs that have no network connection at all.

I want the user to pay for each VM the user runs.

I thought to ask the user for some kind of hardware ID and provide him with serial that is unique for his machine.

But user can just copy the VM image and than have two machines running?

So I thought of having a security dongle. But how can I prevent user from running two VM's on the same machine both connected to the same dongle?

This should be only basic defense so that actual hacking will be required to breach the license and not only spinning one more VM.