Cleanup and performance improvement of the virtio_blk driver

  - Add support for GEOM direct completion. Depending on the benchmark,
    this tends to give a ~30% improvement w.r.t IOPs and BW.
  - Remove an invariants check in the strategy routine. This assertion
    is caught later on by an existing panic.
  - Rename and resort various related functions to make more sense.

MFC after:	1 month