Adding support for Seqpacket

[bstrong04]

Changes

Adding support for the VSOCKSEQPACKET type in VIRTIO devices (specifically VSOCK). Aims to add message handling differences in SEQPACKETs and allow for the creation of socket types / buffers used in event handling.

Still very W.I.P.

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Reason

Currently this is unsupported by firecracker and would allow for stricter message sending protocols for the host and guest to allow for expectations of limited message behavior (one message per receive/send).

Closes #4822

Done in part with @gjkeller.

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License Acceptance

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PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

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• This functionality cannot be added in rust-vmm.