Earlier quoted context omitted.
got a hash of the boot partition, validate when it comes back up.
How do you know that you're not logging in to a vm, with all the data mirrored from your physical server?
Assuming however it could somehow be pulled off; I guess potential ways to mitigate would be to keep a copy of the exact hardware characteristics of the system you originally have, kernel log on bootup, precise size of disks, chipsets of all the various controllers and compare when the system reboots. It would however be possible though extremely hard to get an exact duplicate of all of these at the virtualisation layer level.
You could call the virtualisation layer in the cpu requesting access to virtualise a subsystem, clock the data bus speeds... There should be some overhead from virtualisation that wasn't there when dedicated...
It's an interesting problem. I'll think more about it.