feat(virtio): add VM generation coordination

[appcypher]

TL;DR

Add an event-driven virtio VM-generation device and runtime API that lets a guest acknowledge an exact host-published generation after refreshing kernel randomness.

Description

• add a private virtio-mmio device with separate host request and guest processed fields
• publish a caller-generated 16-byte identifier with a monotonic device-local sequence
• make same-identifier retries idempotent and report replaced or stale requests as superseded
• expose a condition-variable-backed completion handle without retaining the virtio device mutex while waiting
• attach the device after existing devices so their established MMIO and IRQ allocation order remains stable
• provide a runtime builder and VM control surface for installing, publishing, retrying, and waiting for generation processing

Test Plan

• cargo fmt --all -- --check
• cargo test --locked --offline -p msb_krun -p msb_krun_vmm -p msb_krun_devices --lib (130 passed)
• Live macOS ARM64/HVF: pre-boot installation, exact acknowledgement, same-attempt retry, monotonic second update, and stale-request supersession
• Live Linux x86_64/KVM: pre-boot installation, exact acknowledgement, same-attempt retry, monotonic second update, and stale-request supersession