bhyve: Accept a variable-length string name for qemu_fwcfg_add_file.

It is illegal (UB?) to pass a shorter array to a function argument
that takes a fixed-length array. Do a runtime check for names that
are too long via strlen() instead.

Reviewed by: markj
Reported by: GCC -Wstringop-overread
Differential Revision: https://reviews.freebsd.org/D39211