nat/vty: Convert into str for the VTY command

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ERROR: testBSCreload (__main__.TestVTYNAT)
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Traceback (most recent call last):
  File "./vty_test_runner.py", line 656, in testBSCreload
    nat_msc_ip(self, ip, port)
  File "./vty_test_runner.py", line 1096, in nat_msc_ip
    x.vty.command("msc port " + port)
TypeError: cannot concatenate 'str' and 'int' objects

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diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index a4cfb60..f56b576 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -1093,7 +1093,7 @@
     x.vty.command("configure terminal")
     x.vty.command("nat")
     x.vty.command("msc ip " + ip)
-    x.vty.command("msc port " + port)
+    x.vty.command("msc port " + str(port))
     x.vty.command("end")
 
 def data2str(d):