I'm returning Dor's original virtio_driver::wait_for_queue().

The rewrite just masked, with its slightly different timing and redundant
second check before waiting, the real bug which a missing memory barrier
(see separate patch fixing that).

Dor's original code has the good feature that after waking up from a
sleep - when presumably we already have something in the queue - we
check the queue before pessimisticly enabling the host notifications.
So let's use Dor's original code.