Anything that lives outside of userspace.
If one has root and patience - flash BIOS and wait for a coldboot. One can even get an IP stack to pull down new firmware between boots. The user sees a normal post screen and your hypervisor sees normal hardware adapters.
See Jonathan Brossard's 'prior work' slide from his Defcon talk on his work [1] for more details on the state of X86 backdooring.
'Trusted computing' and all that.
[1] http://www.youtube.com/watch?v=yRxDvkKBMTc
https://media.defcon.org/dc-20/presentations/Brossard/DEFCON...