bhyve: fix buffer overflow in QemuFwCfg

We're accessing one element of the newly allocated and the old directory
too much.

Reported by: andy@omniosce.org
Reviewed by: markj
Fixes: 6f9ebb3d0fed2b0ae604dd6daf17f1fe1d8df216 ("bhyve: add helper for adding fwcfg files")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42220