[Rust-VMM] Crate Addition Request: CPU model

[Yi Sun]

## Crate Name

'cpu-model'

## Short Description

A crate to provide a generic framework which has standard interfaces
and flexible mechanism to support customized CPU models.

## Why is this crate relevant to the rust-vmm project?

Customized CPU model is necessary because of below reasons.
1. Avoid CPU hardware vulnerabilities.
2. Keep stable guest ABI.
3. Hard requirement for live migration.