bhyve: emulate graphics stolen memory register

This register contains a host physical address. This address is
meaningless for the guest. We have to emulate it and set it to a valid
guest physical address.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40060