bhyve: read OpRegion address and size for GVT-d

The OpRegion provides some configuration bits and ACPI methods used by
some Intel drivers. The guest needs access to it. In the first step,
we're reading it's address and size.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40040