clean up potential conflict veth before creating new one in create_ns

Signed-off-by: Yonghong Song <yhs@plumgrid.com>
diff --git a/examples/simulation.py b/examples/simulation.py
index 269ad3f..9e2e677 100644
--- a/examples/simulation.py
+++ b/examples/simulation.py
@@ -1,3 +1,4 @@
+import os
 import subprocess
 import pyroute2
 from pyroute2 import IPRoute, NetNS, IPDB, NSPopen
@@ -46,6 +47,9 @@
                 # move half of veth into namespace
                 v.net_ns_fd = ns_ipdb.nl.netns
         else:
+            # delete the potentially leaf-over veth interfaces
+            subprocess.call(["ip", "link", "del", "%sa" % ifc_base_name],
+                            stderr=open(os.devnull, 'w'))
             try:
                 out_ifc = self.ipdb.create(ifname="%sa" % ifc_base_name, kind="veth",
                                            peer="%sb" % ifc_base_name).commit()