bhyve: Validate host PAs used to map passthrough BARs.

Reject attempts to map host physical address ranges that are not
subsets of a passthrough device's BAR into a guest.

Reviewed by: markj, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36238

(cherry picked from commit c94f30ea85b745a5137471876013f79689e0af03)