To install in qemu
Get an install CD or netinst CD.
For graphical install you need nothing special.
For serial install you need qemu-system 0.13 (or the openbios that comes with it in Debian).
Boot with something like qemu -net nic,model=e1000 -net tap,ifname=tap0,downscript=no,script=no -hda debian-ppc.img -cdrom debian-504-powerpc-netinst.iso -boot d -nographic
and on the yaboot commandline (which you won't get with earlier qemu without graphics) you enter install console=ttyPZ0