At that time, /var/log/libvirt/libvirtd.log
(these are also translated)
-----------------------------------------
2013-05-23 04:32:14.588+0000: 11455: error :
virNetDevGetMTU:343 : Cannot get interface MTU on 'ovsbr0': No
such device
2013-05-23 04:32:14.678+0000: 11455: error :
virExecWithHook:419 : Cannot find 'ovs-vsctl' in path: No such
file or directory
2013-05-23 04:32:14.679+0000: 11455: error :
virNetDevOpenvswitchRemovePort:173 : Unable to delete port
(null) from OVS: command does not allowed.
-----------------------------------------
I think above errors comes from install location of OVS as
written in libvirtd.log.