bhyve: use correct size to map the OpRegion

We're already converting the size reported by the OpRegion header from
kilobytes to bytes. We don't have to do that twice when mapping the OpRegion.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45334