MFC r275335:

MFC r275335:

  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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@276487 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f