bhyve: add helper for passthru specific mmio ranges

Intel GPUs have two special memory regions. They are called Graphics
Stolen Memory and OpRegion. bhyve has to emulate both of them. In order
to keep track of those special regions, add generic mmio ranges to the
passthru emulation.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40036