bhyve: avoid side effect in assertion

An assert() was setting the error variable instead of checking it.

Reported by: Coverity Scan
CID: 1521431
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-src/pull/1244

(cherry picked from commit f7d45c5443edc99857fdda19c68301b5ec4a8971)