bhyve: verify OpRegion size

If the OpRegion size doesn't match the size of our igd_opregion struct, it's
using a different layout than we're expecting. To avoid strange issues, we
should exit hard. If we see any devices in the field with a different OpRegion
size, we can analyse it and fix it accordingly.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45335