bhyve: add empty GVT-d emulation

Don't emulate anything yet. Just check if the user would like to pass an
Intel GPU to the guest.

Reviewed by: jhb, markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40038

(cherry picked from commit 90c3a1b6629c80a7e4c3fa86d6b794596fdf06b4)