Persist virtio state and reconstruct console attachments

[ppenna]

Tracking issue: MicroVM support and private snapshot restore.

Motivation

Restoring guest RAM alone is insufficient for a virtio device: negotiated state,
queue ownership, pending work, and interrupt state must agree with the guest.
Console support provides a concrete first consumer of a generic persistence
contract and makes buffered I/O and host-endpoint reconstruction observable.

Proposed Scope

• Define bounded, validated generic virtio device and queue persistence.
• Defer restored device activation until required state and attachments are ready,
  with an explicit acknowledgment of activation success or failure.
• Persist the console's buffered I/O under a documented save-boundary contract.
• Reconstruct host console endpoints rather than serializing process-local handles
  or assuming an endpoint survives capture.
• Include generic persistence and working console reconstruction in the same PR.
• Propagate quiesce and activation failures through the snapshot transaction.

Acceptance Criteria

• Generic state round trips preserve negotiated features, queue state, and
  relevant interrupt/device status; malformed state is rejected without panics.
• No restored device consumes descriptors or emits I/O before activation is
  acknowledged, and a failed activation unwinds cleanly.
• Console buffer tests verify the documented handling of bytes pending at the
  save boundary, including empty and partially consumed buffers.
• A VMM test captures and restores with active console I/O and verifies guest
  progress and the expected byte sequence after endpoint reconstruction.
• Missing or incompatible host endpoints produce a bounded failure and release
  any resources already prepared.

Dependencies

• S03: Base snapshot capture and private restore.
• F04: Deterministic virtio interrupt fixtures.

Non-Goals

Acknowledged activation is a lifecycle requirement. Network and filesystem
persistence are separate consumers of this generic contract.