Don't stop switches that we've already stopped.

Note that this also changes the way that links are deleted;
the reason is that the batch shutdown doesn't currently delete
the links, but OVSSwitch.stop() does. We may wish to revisit
this in the future.

mininet/net.py

@@ -495,20 +495,25 @@ class Mininet( object ):
         if self.terms:
             info( '*** Stopping %i terms\n' % len( self.terms ) )
             self.stopXterms()
-        info( '*** Stopping %i switches\n' % len( self.switches ) )
-        for swclass, switches in groupby(
-                sorted( self.switches, key=type ), type ):
-            if hasattr( swclass, 'batchShutdown' ):
-                swclass.batchShutdown( switches )
-        for switch in self.switches:
-            info( switch.name + ' ' )
-            switch.stop()
-            switch.terminate()
-        info( '\n' )
         info( '*** Stopping %i links\n' % len( self.links ) )
         for link in self.links:
+            info( '.' )
             link.stop()
         info( '\n' )
+        info( '*** Stopping %i switches\n' % len( self.switches ) )
+        stopped = {}
+        for swclass, switches in groupby(
+                sorted( self.switches, key=type ), type ):
+            switches = tuple( switches )
+            if hasattr( swclass, 'batchShutdown' ):
+                swclass.batchShutdown( switches )
+            stopped.update( { s: s for s in switches } )
+        for switch in self.switches:
+            info( switch.name + ' ' )
+            if switch not in stopped:
+                switch.stop()
+            switch.terminate()
+        info( '\n' )
         info( '*** Stopping %i hosts\n' % len( self.hosts ) )
         for host in self.hosts:
             info( host.name + ' ' )