uh_mem: Fix a few oversights and edge cases

[smalis-msft]

This fixes some oversights and edge cases in underhill_mem that were pointed out during an audit of our memory protections code.

1. We need to remove permissions when unaccepting pages on SNP
2. We need to completely forbid changing permissions on individual overlay pages to avoid a potential race condition (does not apply to the default permissions case)
3. We need to be careful around changing the default permissions when pages are locked, since we don't restore default permissions when they're unlocked
4. Remove a stale comment

Fixes #1021

[Copilot]

Pull Request Overview

This PR fixes several security and correctness issues in the underhill_mem memory protection subsystem, addressing edge cases around SNP memory unacceptance, overlay page permissions, and locked page handling.

• Adds permission revocation before unaccepting pages on SNP to close a security window
• Prevents permission changes on individual overlay pages to avoid race conditions
• Refactors locked page checking logic to be more robust

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
openhcl/underhill_mem/src/lib.rs	Main fixes for SNP unacceptance, overlay page validation, and locked page handling logic
openhcl/underhill_mem/src/init.rs	Removes outdated comment about HCL Arc usage

[github-actions]

At least one Petri test failed.