bhyve: avoid a potential deadlock

This unlocks a mutex in an error path, that would otherwise remain locked and
potentially cause a deadlock later on.

Reported by: Coverity Scan
CID: 1521334
Reviewed by: corvink, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45401

(cherry picked from commit 1f3bfc60550cdd019f00bf6d274db762fda4ed2c)