Harden vGPU release ordering in the instance lifecycle

[yummybomb]

Summary

Layer 2 of the vendor VFIO vGPU stack (#366 ← this ← #363 ← #364 ← #321). Where #366 is a behavior-preserving refactor, this layer changes lifecycle semantics — release ordering and retention — still on mdev-era machinery, before the vendor VFIO backend lands above:

• Delete releases the vGPU before other teardown — a successful release persists the cleared assignment before later teardown. A failed release keeps the pre-refactor contract: it is logged and the delete continues, since the release guards never destroy a device they cannot prove is unowned and a skipped release is recovered by startup reconciliation. Stopped instances retry a previously failed release.
• Stop keeps its no-op contract — a failed retained-release on stop logs and retains the assignment metadata instead of failing the stop; assignments are passed to DestroyVGPU as a struct.
• Forks don't inherit retained assignments — a retained (failed-release) vGPU assignment is stripped from fork metadata so a fork can't release its parent's device.
• Start persists a stale release immediately — releasing a stale assignment before start writes the cleared metadata at once, so a later start failure can't leave metadata pointing at a device that's already gone.

Testing

• go build ./..., go vet clean
• go test -race targeted lib/instances lifecycle suites pass

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Note

Medium Risk
Changes instance delete/start/stop ordering and vGPU metadata persistence; incorrect behavior could leak GPU slots or double-release, though tests cover the main paths and failed releases remain best-effort with reconciliation.

Overview
This PR tightens vGPU lifecycle semantics around release ordering, metadata retention on failure, and fork/snapshot correctness, while switching DestroyVGPU to a VGPUAssignment struct.

Delete now blocks the restart policy before teardown (so a partial delete cannot auto-restart a stopped VMM), releases vGPU before network/devices/volumes, and persists cleared assignment metadata when release succeeds. Failed release still logs and continues delete.

Stop no longer clears vGPU metadata when release fails—it retains the assignment for later retry. Calling stop on an already-stopped instance retries releaseRetainedVGPULocked without failing the no-op stop.

Start releases any stale retained assignment up front and saves cleared metadata immediately so a later start failure cannot leave disk pointing at an already-released device.

Forks strip live vGPU assignment fields (profile remains); snapshot restore keeps the instance’s current assignment instead of resurrecting the snapshot’s embedded copy.

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[yummybomb]

Trimmed over-engineering after an architectural review against the parent task (host kernel 6.8 / Ubuntu 24.04 support):

• 1c1f52e — dropped the HasMdev/IsAllocated() compatibility shim. VirtualFunction is internal and never serialized in an API response or persisted, so a single Allocated field is enough.

Kept as-is:

• the delete/stop behavior change (fail and retain assignment metadata when release fails) — orphan reconciliation only runs at startup, so failing the delete is the correct fail-closed behavior;
• the darwin DestroyVGPU unknown-framework error — it keeps the fail-closed rollback semantics identical on macOS, which the lib/instances tests exercise in CI.

Tests: go test -race ./lib/devices, ./lib/resources, ./lib/hypervisor/qemu, ./lib/hypervisor/cloudhypervisor, and the vGPU-targeted ./lib/instances tests all pass; the remaining lib/instances E2E failures require KVM/images and fail identically on an unmodified checkout.

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