This patch defines and manages the syntax:

<domain ....>
 <device>
   <interface type='vde'>
	 ...
	 	<switch path='/tmp/vde.ctl'/>
	 </interface>
 </device>
</domain>

the switch tag can be omitted: vde uses the default switch.
qemu/kvm support: tested.
user-mode linux support is included but not tested yet.
libvirt vde support for virtualbox has not been coded yet.

INSTALL:
download libvirt-0.8.7
..../libvirt-0.8.7$ patch -p 1 < libvirt-0.8.7.vde.patch
..../libvirt-0.8.7$ configure
..../libvirt-0.8.7$ make
..../libvirt-0.8.7$ sudo make install