TCIntf: Don't delete a non-existing root qdisc

In recent kernels, virtual interfaces come without any associated
qdisc, resulting in errors when spawning the network.
Checking for "noqueue" in the tc output, enables to detect that
case and thus avoid deleting the non-existent qdisc.

mininet/link.py

@@ -329,7 +329,7 @@ class TCIntf( Intf ):
 
         # Clear existing configuration
         tcoutput = self.tc( '%s qdisc show dev %s' )
-        if "priomap" not in tcoutput:
+        if "priomap" not in tcoutput and "noqueue" not in tcoutput:
             cmds = [ '%s qdisc del dev %s root' ]
         else:
             cmds = []