link: Fix circular import

Having an 'import mininet.node' in the link module causes
a circular import as the node module also has a top-level
import for the link module.

In order to break the cycle, the import for the node module
is delayed and executed only if using the sole class that
actually needs it in the link module.

mininet/link.py

@@ -26,7 +26,6 @@ Link: basic link class for creating veth pairs
 
 from mininet.log import info, error, debug
 from mininet.util import makeIntfPair
-import mininet.node
 import re
 
 class Intf( object ):
@@ -504,10 +503,12 @@ class OVSLink( Link ):
        than ~64 OVS patch links should be used in row."""
 
     def __init__( self, node1, node2, **kwargs ):
+        from mininet.node import OVSSwitch
+
         "See Link.__init__() for options"
         self.isPatchLink = False
-        if ( isinstance( node1, mininet.node.OVSSwitch ) and
-             isinstance( node2, mininet.node.OVSSwitch ) ):
+        if ( isinstance( node1, OVSSwitch ) and
+             isinstance( node2, OVSSwitch ) ):
             self.isPatchLink = True
             kwargs.update( cls1=OVSIntf, cls2=OVSIntf )
         Link.__init__( self, node1, node2, **kwargs )