Hey folks,
I saw the lastest qemu support pxe so I tried it using the following 
command:

qemu -hda /tmp/a.img -net nic n- user -boot n

Note that now qemu support pxe mode, the mandatory -hda option is no 
more mandatory but this is not directly linked to my patch.
I saw qemu integrates a dhcp & a tftp server but they don't seems to be 
configured/developped for a pxe boot process.
Qemu says "no filename" and exits.

A friend of I asked me "how do you boot using pxe in qemu when you don't 
have a dhcp & a tftp server ?".
So I started to look how we can do that, I modified bootp to catch the 
field 60 of a dhcp request to find if its a pxe request or not.
If yes, I set the filename to "/pxelinux.0", currently this is harcoded, 
maybe you can help me to find a better way to specify a bootloader.

Then, I've patched the tftp server to change the default rootdir in my 
home dir. "~/tftpboot". This is just for a Proof-of-concept, other 
methods could be fine. We can also imagine searching in a path 
restricted to the admin and then in the user dir. So it can give the 
admin the ability of defining some pxe boot for all the vm of a system  
or in the user dir if the user wants to have a special pxe boot scheme.

The last patch I did on the tftp server, is to allow the tsize 
negociation because pxelinux needs it.

This patch apply to 0.9.0, once it is applied and compiled here come the 
test procedure:

create a ~/tftpboot/pxelinux.cfg directory
copy pxelinux.0 in ~/tfptboot/
create a pxelinux configuration file called "default" in 
~/tftpboot/pxelinux.cfg

Et voila ;)

Just call "qemu -hda /tmp/a.img -net nic n- user -boot n", it will start 
pxelinux.

I think this patch is very usefull because it give users & admin the 
ability to use pxe inside the VM without installing the full 
configuration (dhcp,tftp).
This patch is mainly a proof-of-concept, this patch can be improved, 
comments and feedback are welcome.