bhyve: add interface to protect BAR regions of passthru devices

We need an interface for protecting BAR regions to make it easier to use this
feature and to make it usable by external emulations like the GVT-d emulation.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45341