I like that the root on the host OS can assign TUN interface to users 
and that the root configure it. So the user only use the new -tun 
<devname> option to use the preconfigured TUN interface without the need 
to execute a trick and unsecure script.

Here is the added desciption:
----------------------------------------------------------------------
-tun devname	
     Try to use devname while opening a tap/tun host network interface 
and use it. If it work, the network init script is not executed for this 
interface. If it don't work, the interface will use the name assigned by 
the operating system and the network init script is executed.

      This option permit the use of preconfigured interface. For 
example, as root you can assign a tun interface to a user and configure 
it like this:

            tunctl -u bob -t tun2
            ifconfig tun2 192.168.2.1

      Then bob can use this interface with the option "-tun tun2". Note 
that option permit the use of a DHCP server on the host to configure the 
guest interface.
----------------------------------------------------------------------

I have also patched the qemu-doc.html but cvs diff don't work with it.

I have tryed to be compatible with the -tun-fd option, but this is 
untested. Also the tun_open() function for BSD system will probably not 
handle this case as expected.
-- 
Jean-Christian de Rivaz