MFC r238072,r238360,r240430,r241469,r241470,r241495,r244136,r244200

Sync VirtIO with HEAD.

 - r238072: Do not include <sys/types.h> in the local headers.
 - r238360: Various VirtIO improvements
 - r240430: No need to leak these into the includer's namespace.
 - r241469: virtqueue: Fix non-indirect virtqueues
 - r241470: Add Virtio SCSI driver
 - r241495: Fix build with PAE enabled
 - r244136: Remove duplicated lines
 - r244200: Start taskqueues threads after attach cannot fail

Approved by:	grehan (mentor)
MFC r247829

    Only set the barrier flag if the feature was negotiated

Approved by:	grehan (mentor)
MFC r251769

  Fix setting of the Rx filters

  QEMU 1.4 made the descriptor requirement stricter - the size of buffer
  descriptor must exactly match the number of MAC addresses provided.