Decouple Vmm from its Firecracker-specific RPC interface

[acatangiu]

This issue is part of a story that tracks a state where the vmm crate is no longer Firecracker-specific.

Benefits:

• swapping out Vmm implementations in Firecracker - the vmm crate used in Firecracker could be one from rust-vmm.
• upstreaming/outsourcing the current Vmm implementation in Firecracker to other projects.
• reducing the barrier-to-entry in Firecracker code-base by having much more contained components.
• clear language-enforced dependencies in the form of crates helps keeping the code clean by forcing contributors to work at the right level with the right resources.

Scope:

vmm_config, VmResources, Pre-boot and Runtime Controllers and rpc_interface to be moved to an external rpc_interface crate that depend on Vmm. Dependency graph will be:

+---------------+     Dependencies
|  Firecracker  |           |
+---------------+           |
|  API Server   |           |
+---------------+           |
| RPC Interface |           |
+---------------+           |
|      Vmm      |           |
+---------------+           V

With Vmm decoupled from all Firecracker-specific configuration primitives and logic.

[acatangiu]

Depends on:

• vmm: unify VmmController with RuntimeApiController #1776
• move vmm_config under rpc_interface #1777
• move resources under rpc_interface #1778

[luminitavoicu]

We don't have an immediate plan for this. Will reopen in the future if necessary.