[Hardening aspect] EFI boot stub audit & hardening

[ereshetova]

Problem

If a CoCo guest is booted using drivers/firmware/efi/libstub, this code needs to be audited, fuzzed and hardened to withstand malicious inputs from host/VMM. In particular some components of efi boot stub are actively receiving inputs from the host, such as Graphics Output Protocol.

Solution

Audit, fuzz & harden the code.