ref: c613e336df8eed2b952acf2cb28e865e92e2daf7
dir: /sys/src/boot/qemu/boot.txt/
mw.b 0x40010000 0x0 0x10000 load ${devtype} ${devnum}:${bootpart} 0x40010000 ${prefix}plan9.ini load ${devtype} ${devnum}:${bootpart} 0x40100000 ${prefix}9qemu.u && bootm 0x40100000