#4547: Refactor vmm builder code to simplify logic that creates the microVM to boot

[andr3wy]

Changes

-Refactor code building VMM to be more modular and reduce compile time conditionals.
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Reason

-Currently, the logic to create a Vmm object is scattered across vmm/lib.rs and vmm.builder.rs files and it is quite convoluted and some times difficult to follow. Moreover, there is a lot of architecture specific code inserted in arbitrary places which further increases the un-readability. Apart from the aesthetics and readability aspect, the state of the code makes it quite difficult to unit test.

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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.

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• This functionality cannot be added in rust-vmm.

[JonathanWoollett-Light]

Related to #4411

[roypat]

Hi @andr3wy,
thanks for having a go at this issue. Since this is still in a very early draft state, and hasn't seen any update in a few months I'll go ahead and close this for now, so that potentially another contributor can pick this task up in the future. Please feel free to reopen it though if you want to come back to it at some point!