Ingram Micro asked Microsoft and the answer was:
"For Windows Servers in a cluster, any CPU on any server on which a VM might ever run in that cluster must be licensed. We can not determine for the customer whether any limiting technology would be compliant. The customer must license each server in a cluster for Windows Server VM use.
SPLA Compliance Team"
...which implies some level of "limiting technology" might be compliant, or they wouldn't have said that.