MFC r273515, r274055, r274063, r274215, r274065, r274502:

Add VirtIO console driver.
MFC r274325:

  Enable LRO by default when available on vtnet interfaces

  The prior change to not enable LRO by default has confused several
  people. The configurations where LRO is problematic is not the
  typical use case for VirtIO, and due to other issues, this often
  requires checksum offloading to be disabled anyways.