What's a unique, persistent alternative to MAC address?

What's a unique, persistent alternative to MAC address?

I need to be able to repeatably, non-randomly, uniquely identify a server
host, which may be arbitrarily virtualized and isn't under my control.
A MAC address doesn't work because in some virtualized environments,
network interfaces don't have hardware addresses.
Generating a state file and saving it to disk doesn't work because the
virtual machine may be cloned, thus duplicating the file.
The server's SSH host keys may be a candidate. They can be cloned like a
state file, but in practice they generally aren't because it's such a
security problem that it's a mistake not often made.
Are there any alternative identifiers for a host?