virtio: Add support for virtio packed descriptor queues and make them the default.

[Brian-Perkins]

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[github-actions]

⚠️ Unsafe Code Detected

This PR modifies files containing unsafe Rust code. Extra scrutiny is required during review.

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[Copilot]

Pull request overview

This PR adds virtio packed descriptor queue support to the virtio core queue implementation, advertises packed queues by default in PCI/MMIO transports, and expands the virtio test suite to validate both split and packed queue behavior.

Changes:

• Advertise VIRTIO_F_RING_PACKED by default for virtio PCI/MMIO devices (while retaining split queues as fallback when not negotiated).
• Refactor split-queue logic into its own module and introduce a new packed-queue implementation.
• Update/extend unit tests to exercise both split and packed queue paths and adjust expected feature bits.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
vm/devices/virtio/virtio/src/transport/pci.rs	Advertises packed ring feature (ring_packed) by default for PCI transport.
vm/devices/virtio/virtio/src/transport/mmio.rs	Advertises packed ring feature (ring_packed) by default for MMIO transport.
vm/devices/virtio/virtio/src/tests.rs	Updates feature expectations and adds coverage for packed queues alongside split queues.
vm/devices/virtio/virtio/src/spec.rs	Extends queue spec types for packed descriptors/event suppression.
vm/devices/virtio/virtio/src/queue.rs	Selects split vs packed queue implementation based on negotiated features; refactors split logic out.
vm/devices/virtio/virtio/src/queue/split.rs	New module containing the split queue get/complete logic previously in queue.rs.
vm/devices/virtio/virtio/src/queue/packed.rs	New packed queue get/complete implementation.

[github-actions]

At least one Petri test failed.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

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[jstarks]

Minor suggestion on the descriptor chain walk — see inline comment.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.