== DF-3959 vtnet_exec_ctrl_cmd unbounded ctrl-vq poll ==
wedged device (used->idx never advances):
  kernel loop spun 200000000 times and never completed (harness cutoff hit) — in the kernel this loop has NO cutoff: if_vtnet.c:2407 -> virtqueue_poll -> virtqueue.c:578 spins forever
  caller holds the ifnet serializers (ASSERT_IFNET_SERIALIZED_ALL, if_vtnet.c:2380) => unkillable kernel thread wedge / CPU burn
  no timeout, no tsleep, no kthread_exit, no SIGKILL escapes a kernel spin loop
wedged device, FIXED driver: poll gives up after 10000 iterations (~1s), ack stays VIRTIO_NET_ERR -> caller gets EIO, thread survives: timed out cleanly