If you want to boot latest version of Slitaz:
qemu-system-x86_64 -device virtio-net-pci,netdev=net0 -netdev user,id=net0,tftp=$HOME/configs/tftp/,bootfile=http://mirror.slitaz.org/pxe/ipxe.kpxe
or many other OSes.
qemu-system-x86_64 -device virtio-net-pci,netdev=net0 -netdev user,id=net0,tftp=$HOME/configs/tftp/,bootfile=http://boot.netboot.xyz/ipxe/netboot.xyz.kpxe