MFC 347033:
    Increase the VirtIO segment count to support modern Windows guests.
    
    The Windows virtio driver ignores the advertized seg_max field and
    assumes the host can accept up to 67 segments in indirect descriptors,
    triggering an assert in the bhyve process.
    
    This brings back r282922 but with a couple of changes:
    - It raises the block interface segment limit to 128 instead of 67.
    - Linux's virtio driver assumes that the segment limit is no
      larger than the ring size.  To avoid breaking Linux guests,
      raise the VirtIO ring size to 128, and cap the VirtIO segment
      limit at ring size - 2 (effectively 126).