Comment 3 for bug 247677

It looks as if you don't have non-root permissions to use the tun device. Do the following:

$ sudo addgroup tun
$ sudo adduser $(id -un) tun

log-out and log-in again to effect the new group membership, then check "tun" is listed in the user's groups:

$ groups

Now follow the instruction in my 'Virtual Machines with VDE Networking' tutorial, in the section "Give New Devices tun Group Ownership"

http://tjworld.net/wiki/Linux/Ubuntu/VirtualMachinesWithVDENetworking#GiveNewDevicestunGroupOwnership

You can manually set the permissions to avoid requiring a system restart:

$ sudo chown :tun /dev/net/tun
$ ls -l /dev/net/tun
crw-rw---- 1 root tun 10, 200 2008-07-02 10:49 /dev/net/tun

Now try the virt-manager procedure again.

You *might* need to reboot to ensure the permissions are set correctly by the udev rule in the future.