EDK2's QemuBootOrderLib inspects the bootorder file provided
via fw_cfg and requires it to be NUL-terminated. Otherwise,
it rejects the supplied bootorder and falls back to its
default boot order.
Currently, bhyve registers bootorder with qemu_fwcfg_add_file()
using bootorder_len returned by open_memstream(), which excludes
the trailing NUL byte.
Fix that by passing bootorder_len + 1 to qemu_fwcfg_add_file() so
the fw_cfg payload is properly NUL-terminated.
PR: 279720
Found with: codex (gpt-5.6-sol)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation