feat: Enable gdb debugging on x86

[JackThomson2]

This PR is adding GDB support into Firecracker. This is will allow us to connect GDB to the guest kernel and enable step through debugging.

This is achieved by leveraging https://github.com/daniel5151/gdbstub which allows us to debug the kernel over the GDB Remote Serial Protocol. The way this is managed is through a new GDB thread which is used to manage the interaction between the VCPUs and the GDB server.

A list of the current features supported in x86

• Multithreaded Debugging (multi core VM's)
• Register Read/Write
• Guest Memory Read/Write
• Software Breakpoints Add/Remove
• Hardware Breakpoints Add/Remove
• Single Step
• Continue

The implementation is feature gated behind the debug flag which aims to allow us to remove all the dependencies at compiletime

Changes

• Adding GDB support for guest kernel debugging
• Added new debug feature to firecracker which when enabled will start the GDB server and block the boot until GDB connects and resumes the execution

Reason

• Allow us to debug the guest kernel with GDB

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

Attention: Patch coverage is 33.72093% with 57 lines in your changes missing coverage. Please review.

Project coverage is 83.96%. Comparing base (36ecc4f) to head (898ee9a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vmm/src/gdb/arch/aarch64.rs	4.76%	40 Missing ⚠️
src/vmm/src/builder.rs	55.26%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4797      +/-   ##
==========================================
- Coverage   84.38%   83.96%   -0.43%     
==========================================
  Files         249      250       +1     
  Lines       27496    27758     +262     
==========================================
+ Hits        23203    23306     +103     
- Misses       4293     4452     +159     

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[ShadowCurse]

Please squash 1 and 2 commits.

[roypat]

I've had a very haphazard look at it. Mostly it's just nits about Rust style things. The one thing I'd like to discuss is the passing of Tids all over the place. I outlined in a comment how we might be able to avoid this. Could you have a look at that? :o

clicked the wrong button, sorry

[ShadowCurse]

LGTM

[roypat]

Overall, looks good to me, good job! No concerns really, just a bunch of small things. I guess the biggest open question is how to deal with someone doing cargo build --feature gdb --target aarch64-unknown-linux-gnu. At least it should give a nice error message about that not being supported.

[kalyazin]

doc LGTM