Add an end-to-end cold-boot microVM profile

Tracking issue: MicroVM support and private snapshot restore.

Motivation

Small, purpose-built guests need a usable OpenVMM machine option with a compact,
documented boot and device contract. The first user-facing PR should provide a
complete cold-boot workflow so reviewers can run the profile, inspect its guest ABI,
and validate it without assembling unfinished feature pieces.

Proposed Scope

• Define the microVM machine contract: PVH entry, guest memory layout, fixed-MMIO
  devices, interrupts, and supported platform capabilities.
• Wire the profile through machine construction, storage configuration, CLI, and
  RPC, with consistent validation of unsupported combinations.
• Build a small PVH test guest from source and make it discoverable through the
  normal Petri artifact workflow.
• Add cold-boot VMM tests and the corresponding Flowey CI source changes, then
  regenerate generated workflows.
• Document supported configurations and runnable CLI/RPC examples in the Guide.
• Keep the complete cold-boot feature, guest artifact, tests, docs, and CI in one
  PR rather than exposing a partially usable machine option.

Acceptance Criteria

• The source-built guest cold-boots and reports success through the configured
  device path using both supported CLI and RPC entry points.
• Tests verify the advertised memory, device, interrupt, and storage contract.
• Invalid options and incompatible profile/device combinations fail before
  guest execution with actionable errors.
• Applicable KVM and WHP VMM tests run through cargo xflowey vmm-tests-run,
  with unavailable backend or architecture coverage recorded explicitly.
• CLI parsing, help, documentation, and lint checks pass with the new options.
• Existing machine types retain their defaults and boot behavior.

Dependencies

• B01: Minimal chipset and fixed-MMIO primitives.
• F06: RBX plumbing and validated PVH loading.

Non-Goals

This issue establishes cold boot, not snapshot capture, device restore, live
migration, or general-purpose firmware support for the new profile.

Start with the linked 00-microvm.md tracking document and dependencies B01 (#4434) and F06 (#4427); the issue names no implementation files. Trace the machine, CLI, RPC, Petri artifact, Flowey, and Guide entry points, then run cargo xflowey vmm-tests-run. Done means a source-built PVH guest cold-boots through CLI and RPC, contract and validation tests pass, workflows regenerate, and documentation and existing machine behavior remain correct.