Provide deterministic virtio interrupt fixtures

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A fixed-MMIO machine and persistent virtio devices need precise tests for interrupt
delivery. Fixtures that expose interrupt transitions deterministically allow
transport, chipset, and restore changes to be tested without relying on a running
VM or host scheduling to observe an interrupt.

Proposed Scope

• Provide reusable test fixtures that record and expose relevant interrupt
  transitions through the existing virtio interrupt interfaces.
• Let tests control observation and reset explicitly while preserving the
  interrupt semantics expected by each interface.
• Use the fixtures in focused transport tests as a baseline for later chipset and
  persistence work.
• Keep these fixtures independent of transport performance optimizations.

Acceptance Criteria

• Tests can deterministically observe interrupt assertion, deassertion, or
  notification as appropriate to the interface under test.
• Repeated notifications and fixture reset do not leak state between tests.
• The fixtures work without a live hypervisor and pass on supported test hosts.
• Existing virtio behavior remains unchanged, and the tests can detect a
  deliberately missing or unexpected interrupt transition.
• Fixtures test interrupt semantics without requiring an accelerated transport
  path.

Dependencies

No feature prerequisites.

Non-Goals

This issue does not change the device transport, introduce a new interrupt
optimization, or implement snapshot persistence.