bhyve: correctly remove a memory mapping

In a normal code path while setting up GPU passthrough, the size
parameter to munmap() is wrong and its operation not checked for errors,
therefore leaking resources.

Reported by: Coverity Scan
CID: 1519830
Sponsored by: The FreeBSD Foundation

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1262