nvme_driver: split the mesh channel to the queue handler between commands and requests (#2708)

[gurasinghMS]

Clean cherry pick of PR #2708

Currently, in QueueHandler::poll_fn, we register a waker with the receiving channel only when there is space available in the submission queue (SQ). This channel multiplexes two kinds of traffic: requests that write to the submission queue, and control‑plane requests for the handler.
When the submission queue is full, poll_fn does not register a waker with recv. Instead, it relies entirely on incoming completions and interrupts to wake the task. As a result, if completions are slow, not only is command submission affected, but control‑plane operations such as Save, Inspect, and NextAen are also delayed—even though they are unrelated to SQ writes.
With this change, the run loop now listens on three separate channels:

1. A command channel for requests that write to the submission queue
2. A control‑plane channel for operations like Save and Inspect
3. A completion channel

This separation ensures that control‑plane operations can continue to make progress even when the submission queue is full and completions are slow. In particular, Save() should no longer be blocked by SQ backpressure.
Note: I am still working on a VMM test to validate behavior when the submission queue is full.

[Copilot]

Pull request overview

This PR refactors the NVMe queue pair’s internal messaging so that control‑plane operations (like Save and Inspect) are no longer backpressured by a full submission queue or slow completions, improving responsiveness under load.

Changes:

• Splits the previous single mesh channel between QueuePair and QueueHandler into two channels: one for control‑plane requests (Req) and one for data‑plane commands (Cmd), updating QueueHandlerLoop and Issuer accordingly.
• Refactors QueueHandler::run to poll the command channel only when there is SQ and pending‑command capacity, while always polling the control‑plane channel and giving it priority over completions.
• Updates inspection wiring (#[inspect(flatten, with = ...)]) and save/restore paths to use the new control‑plane channel (send_req / recv_req) without changing external APIs.

[Copilot]

Comment uses the acronym "NVME" while the rest of this module and NVMe spec consistently use "NVMe"; please fix the casing here for consistency (e.g., "NVMe commands").

Suggested change

	// Look for NVME commands
	// Look for NVMe commands

[mattkur]

Has been tested in production and in local vmm_tests (that change is still iterating in main).