fix(vsock): save state after sending a notification

[ShadowCurse]

Changes

This is a fix for a fix introduced in #4796
The issue was in vsock device hanging after snapshot restoration due to the guest not being notified about the termination packet. But there was bug in the fix: we were saving the vsock state before the notification was sent, thus discarding all modifications made to the queue in order to sent the notification.

The reason original fix worked, is because we were only testing with 1 iteration of snap/restore. This way even though we lose synchronization with the guest in the event queue state, it worked fine once. But doing more iterations causes vsock to hang as before.

This PR fixes the issue by saving the vsock state after the notification is sent and modifies the vsock test to run multiple iterations of snap/restore.

Reason

Fixes: #4811

License Acceptance

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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

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