bhyve: Fix vtscsi maximum segment config

The seg_max value reported to the guest should be two less than the
host's maximum, in order to leave room for the request and the
response. This is analogous to r347033 for virtio_block.

We hit the "too many segments to enqueue" assertion on OneFS because
we increase MAXPHYS to 256 KB.

Reviewed by: bryanv
Discussed with: cem jhb rgrimes
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20529