Ironic: Do not try to unplug VIF if not associated

During instance spawn, Ironic attempts to unplug any plugged VIFs from
ports associated with an instance. If there are no associated VIFs
to unplug, this would raise an exeception that would be logged into
n-cpu. This patch fix that behavior by making the driver check if the
port has an VIF associated with it before trying to remove it.

Change-Id: I5608b9eec625452edda9a0c9eb366ece2a95800f
Closes-Bug: #1292733
Merge "Ironic: Do not try to unplug VIF if not associated"