vCPU Hot-unplug

[ghost]

Changes

• Implement the inverse operation of vCPU hot-plugging - vCPU hot-unplugging.
• Changed the /hotplug endpoint, now the user requests a target number of vCPUs to be reached.

Unfinished

• Currently only the guest portion has been completed, an API call can be made, and the vCPUs will be successfully removed from the guest
• The host portion has not been completed. The backing threads need to be deleted after the guest has ejected the CPU. This requires a refactor of vCPU handles, to enable the CpuContainer the ability to terminate threads.

Reason

To provide the inverse of hot-plugging, so that VMs can be scaled in both directions.

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.21%. Comparing base (a541725) to head (99858de). Report is 1 commits behind head on feature/vcpu-hotplug.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           feature/vcpu-hotplug    #4753      +/-   ##
========================================================
- Coverage                 81.95%   79.21%   -2.74%     
========================================================
  Files                       257      224      -33     
  Lines                     31762    26485    -5277     
========================================================
- Hits                      26031    20981    -5050     
+ Misses                     5731     5504     -227     

Flag	Coverage Δ
5.10-c5n.metal	?
5.10-m5n.metal	?
5.10-m6a.metal	?
5.10-m6g.metal	79.21% <100.00%> (+<0.01%)	:arrow_up:
5.10-m6i.metal	?
5.10-m7g.metal	79.21% <100.00%> (+<0.01%)	:arrow_up:
6.1-c5n.metal	?
6.1-m5n.metal	?
6.1-m6a.metal	?
6.1-m6g.metal	79.21% <100.00%> (+<0.01%)	:arrow_up:
6.1-m6i.metal	?
6.1-m7g.metal	79.21% <100.00%> (+<0.01%)	:arrow_up:

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