Comment 1 for bug 944168

At a glance, it seems that unplug() being called twice, with the second call failing because the first call successfully unplugged the vif already.

The unplug() action on the bridge does nothing, so there's no harm in calling it twice, which may explain why this is only seen with OVS.

A quick workaround would be to remove the "raise" line at the end of the unplug() method for LibvirtOpenVswitchDriver, in which case you will see the log message still, but it should not disrupt the reboot (this is probably better behavior as well, regardless of the true cause of the bug).