bhyve: copy OpRegion into guest memory

This makes the OpRegion accessible by the guest. However, the guest
doesn't know the address of the OpRegion. This will be fixed by an
upcoming commit.

The range of the OpRegion is added to the e820 table. This allows the
guest firmware to easily pick up this range and to reserve it properly.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40041