microvm: at a platform restore, attach an aggregate export to the dormant filesystem slot #540

Parent: #835 (#833). Unblocks: #857.

Goal. A platform template captures a dormant filesystem slot. At restore, OpenVMM attaches to it either one directory, as today, or an aggregate of host directories and files, validated as at a cold launch, before any vCPU runs. Clones then export what cold boots export.

Today (NVX's pin, 79a501b6):

• The slot's backend is an aggregate, one directory or dormant (openvmm/openvmm_entry/src/microvm/config.rs:570-628). A cold launch validates an aggregate's children: their kinds, access modes, hidden, re-exposed and writable paths, the owner mode, and limits (openvmm/openvmm_entry/src/cli_args/microvm.rs:441-542, :1066-1117).
• A snapshot that captured an aggregate restores only with the same children, names and modes (openvmm/openvmm_entry/src/microvm/filesystem.rs:997-1027).
• A dormant slot accepts only --mount at restore (:983-995), and a test checks that an aggregate is refused (:2008-2017).

Expected behavior:

• At a restore of a platform snapshot whose slot is dormant, --mount-aggregate with its --mount-child options, and their --mount-deny, --mount-allow, --mount-write and --mount-owner policy, attach an aggregate to the slot, validated as at a cold launch.
• No device is added: the slot's device ID, tag, MMIO, IRQ, features and queues stay as captured. The saved command line doesn't change, and OpenVMM exposes no layout: the guest learns of the export over its own control channel.
• --mount, or no attachment, behaves as today. A snapshot that captured an attachment keeps its exact-match rules, and workload and instance snapshots are unchanged.
• Refused: an aggregate on a snapshot without the dormant slot's version, and children that a cold launch would refuse.

Tests: VMM tests that restore a dormant slot with an aggregate of directory and file children, with mixed modes and policies; invalid aggregates, refused; snapshots with a captured attachment, and older snapshots, unchanged.

NVX pin: a PR that moves NVX's OpenVMM pin and adds a VMM-level snapshot scenario, as for the other changes in #835.

Replaces this issue's earlier plan: no guest-observable layout, no attachment receipts (#548), and no dependency on #543 (closed) or #544 (the performance workstream).