Provide minimal chipset and fixed-MMIO building blocks

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A lightweight machine profile needs to compose a small chipset and place virtio
devices at a defined MMIO layout without inheriting unnecessary platform devices.
Reusable chipset and transport primitives would let the machine contract be added
separately from the plumbing that makes its layout and interrupts possible.

Proposed Scope

• Add the generic chipset composition and fixed-MMIO registration primitives
  needed by a minimal machine profile.
• Make address ranges, device placement, and interrupt routing explicit and
  validate incompatible or overlapping assignments.
• Adapt affected constructors and exhaustive matches in the same PR, including
  the ACPI IRQ initialization required by any new interface.
• Preserve defaults and behavior for existing chipset users.
• Exercise interrupt behavior with deterministic fixtures rather than depending
  on the complete machine profile.

Acceptance Criteria

• A focused fixture constructs a minimal chipset with deterministic device
  addresses and interrupt routing.
• Invalid ranges, overlaps, and unsupported assignments are rejected before
  devices become accessible.
• MMIO access and interrupt tests exercise the new primitives without a live
  hypervisor or the cold-boot feature.
• Affected Windows and Linux constructors and match arms compile without
  requiring a later PR to supply missing initialization.
• Existing chipset tests retain their behavior, and the new APIs document
  their layout and ownership constraints.

Dependencies

• F04: Deterministic virtio interrupt fixtures.

Non-Goals

This issue does not expose a new CLI machine type, choose the full cold-boot
contract, or add snapshot persistence.

Start with the existing chipset constructors, exhaustive match arms, ACPI IRQ initialization, and chipset tests; trace how current device placement and interrupts are represented. Use the deterministic virtio interrupt fixture from #4425 as a dependency, then add focused tests for fixed MMIO, invalid assignments, and interrupt behavior. Done means the minimal primitives work without a live hypervisor while existing chipset behavior remains unchanged.