IVSSwitch: support batch shutdown

Not a single command like the OVS one, but it still greatly decreases the
shutdown time.

This does assume that stop() is called after batchShutdown(), which is true in
the current mininet code.

mininet/node.py

@@ -1106,6 +1106,13 @@ class IVSSwitch(Switch):
                    'not be loaded. Try modprobe openvswitch.\n' )
             exit( 1 )
 
+    @classmethod
+    def batchShutdown( cls, switches ):
+        "Kill each IVS switch, to be waited on later in stop()"
+        for switch in switches:
+            if type(switch) == cls:
+                switch.cmd( 'kill %ivs' )
+
     def start( self, controllers ):
         "Start up a new IVS switch"
         args = ['ivs']