MFC r267520, r267521, r267522, r267523, r267524:

  - Remove two write-only local variables
  - Remove unused element in the vtnet_rxq structure
  - Remove kernel specific macro out of the VirtIO PCI header file
  - Move the VIRTIO_RING_F_* defines out of virtqueue.h into
    virtio_config.h
  - Make the feature negotiation code easier to follow
  - Force two byte alignment for all control message headers