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fix(virtio): disable queues before clearing their registers on reset

VirtioPciRegisterMmio::reset() cleared the ring addresses of each
queue and only then set enabled to false. A worker thread reading the
registers concurrently, e.g. to set up queues on activation, could
see an enabled queue whose addresses were already zeroed, and place
the rings at GPA 0.

Store enabled = false first, and make SplitQueue::new() and
PackedQueue::new() load the ring registers before re-checking enabled.
A reader that observes any cleared register then also observes the
queue as disabled, so a partially cleared queue is never accepted.

This does not cover a driver that re-enables the queue with new
addresses before the worker handles the reset. Workers must not
re-read the registers after activation.

Assisted-by: Antigravity:Claude-Opus-5
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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