bhyve: protect MMIO mapped BDSM register

The Windows graphics driver reads the value of the BDSM register from MMIO
space. This value makes no sense in our virtual environment because it's a host
address. Therefore, we have to trap and emulate it.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45342