bhyve: pass address of OpRegion to the guest

Don't allow access to the physical ASLS register. It contains a host
address which is meaningless for the guest. Additionally, it allows the
guest to safely rewrite this register.

This is the last commit required for GVT-d. Nevertheless, it might not
work due to missing firmware support.

MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D26209