nvme_driver: blockers to adding a save/restore keepalive unit test

Tracking the set of things that block adding a unit test for NvmeDriver::save() → NvmeDriver::restore() with keepalive.

Blockers

1. PagePool not test-friendly for save/restore — PagePool::restore() requires exact backing size matching, which is hard to achieve when the save and restore sides use different TestMapper instances. The pool needs to be refactored to support test save/restore cycles, or a mock DmaClient needs to provide PFN-stable blocks via attach_pending_buffers().

2. Restored driver hangs on shutdown due to stale admin queue — After restore with re-allocated DMA blocks, the admin queue's pending AER command never completes because the controller's SQ/CQ BAR addresses are stale. shutdown() waits forever. There's no force-shutdown or AER cancellation mechanism for tests.

Sub-issues track individual blockers.