The platform root-of-trust is in the real, motherboard mounted TPM. AMD's "Pacifica" or Intel's VT/LT based platforms allow to generate "soft TPMs" for each virtualized compartment/space. Please check out slides 11 through 17
And the one thing that everybody needs to keep in mind is that every TPM, real or soft, wiill need the trust infrastructure.
Virtualization is a good thing!
Here the link to a quite explanatory presentation. It has been posted before...