Less memory checks in the queue

[ShadowCurse]

Changes

Replace accesses to the queue objects reside in the guest memory from always using GuestMemoryMmap to storing pointers to them directly. The reason we can do this is that these objects do not move in the guest memory, so storing direct pointers to them is safe.

Reason

This optimization avoid many memory check when accessing guest memory and thus can improves performance of all virtio devices.

License Acceptance

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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.