CPUID should return only information provided as part of the attestation

[davidchisnall]

Currently, CPUID is emulated using an ocall, which allows an attacker to provide any information that they want. We should provide a set of CPU features in the initial configuration (subject to attestation) and whitelist a set of leaf nodes that we'll emulate. In particular, it's important that we don't return the wrong values for current core ID and instead map this to ethread ID.

The same information should be used to populate /proc/cpuinfo (#529)