fix: Update `api_event_fd` in `PrebootApiController`

[JonathanWoollett-Light]

Changes

Updates and explicitly documents purpose of the api_event_fd in PrebootApiController::build_microvm_from_requests. The functional change should minimize latency between pre-boot API request handling as it removes 1 syscall that was previously being done between every request.

Reason

The handling of api_event_fd could have previously given the false impression it played a functional role in the pre-boot API.

License Acceptance

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PR Checklist

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

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• [x] This functionality cannot be added in rust-vmm.