if_vtnet - Allocate struct vtnet_tx_header entries from a queue.

if_vtnet - Allocate struct vtnet_tx_header entries from a queue.

* The existing code was sequentially allocating from an array of
  struct vtnet_tx_header, using an appropriately sized area.
  But this scheme turns out to be a very bad idea when we get out-of-order
  completions in the virtqueue.

* Instead allocate struct vtnet_tx_header entries from an SLIST.

* This should avoid crashes from memory-corruption or use-after-free in
  if_vtnet(9), when running in KVM on Linux, using Linux's vhost-net
  in-kernel virtqueue accelerator.