Persist FUSE and virtio-fs state with attachment revalidation

[ppenna]

Tracking issue: MicroVM support and private snapshot restore.

Motivation

The guest-visible FUSE session and virtio-fs queues cannot be reconstructed safely
by reconnecting a host directory alone. Restored protocol state must agree with
the exported filesystem and its access policy, and stale host handles must not
silently identify a different file or grant a broader attachment.

Proposed Scope

• Persist the FUSE session and virtio-fs device/queue state required by the
  supported snapshot contract.
• Define treatment of node identities, open handles, and pending requests at the
  capture boundary, rejecting unsupported states explicitly.
• Reconstruct and revalidate live filesystem attachments, including export roots,
  access modes, and the identity requirements of restored protocol state.
• Use the shared attachment-preparation contract introduced for networking and
  participate in its acknowledged activation and cleanup sequence.
• Document the consistency assumptions for externally mutable host filesystems.

Acceptance Criteria

• Focused tests round-trip supported FUSE and queue state and reject malformed
  or incompatible state without panics.
• Replaced, missing, or incompatible host objects fail revalidation rather
  than silently acquiring another object's identity or broader permissions.
• A VMM test captures and restores during active filesystem I/O, then verifies
  data access, request completion, and the documented handle semantics.
• Failure during attachment reconstruction releases opened resources and does
  not activate a partially restored device.
• Combined console, network, and filesystem restore is exercised through
  cargo xflowey vmm-tests-run; unit coverage alone does not close this issue.
• The Guide states which live-attachment states are supported and what host
  filesystem changes invalidate a restore.

Dependencies

• D02: Network persistence and shared attachment preparation.

Keep this PR after network reconstruction so both devices use the same preparation
and failure contract.

Non-Goals

This issue does not snapshot the host filesystem itself or provide point-in-time
consistency for arbitrary external writers. Dormant devices and choosing a new
filesystem attachment at restore time are separate work.