kernel -- virtio: Implement virtio-layer lwkt_serializer for devices

kernel -- virtio: Implement virtio-layer lwkt_serializer for devices

* Switch virtio device targets (currently only -blk) to use a
  serializer for synchronizing per-target state. Thread the
  serializer through virtqueue_notify (release around kick) and
  into the bus interrupt setup code.

* Remove dump support from virtio-blk for now; it doesn't work and
  results in further panics from an incorrectly setup bio.

* Other cleanups of virtio-blk (de-indirect locks).