bhyve: return EEXIST when adding a fwcfg item twice
Reviewed by: rew
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39338
(cherry picked from commit e37edc91b6a0a0116005bdd0fff7c911c37adabf)