Probably, the only option for you is to build libvirt from source, unless you are ok with using Fedora 17 Alpha (I believe this is the only mainstream distro that has the bleeding libvirt version, but I could be wrong on this). It should not matter what kind of OVS version you are using. The only restriction is that ovs-vsctl utility (from Open vSwitch) must be in the $PATH, because libvirt needs to call it.
Currently only bridged networks are supported, so you must create the OVS bridge beforehand:
ovs-vsctl add-br ovsbr
The other thing you need to do is edit the Domain XML file:
<interface type='bridge'>
<mac address='52:54:00:d0:3f:f2'/>
<source bridge='ovsbr'/>
<virtualport type='openvswitch'>
</virtualport>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
And then (re)start the VM.
To verify, if libvirt attached VM interface to the ovsbr as expected, you should run the following command and look for vnetX interface:
ovs-vsctl show
Hope this helps.