Avoid a warning from OpenBSD linker

Avoid the warning below by using snprintf:
../libhw64/vl.o(.text+0x78d4): In function `get_boot_devices_list':
/src/qemu/vl.c:763: warning: sprintf() is often misused, please use snprintf()

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>