trace: [tracetool] Explicitly identify public backends

Public backends are those printed by "--list-backends" and thus considered valid
by the configure script.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/scripts/tracetool.py b/scripts/tracetool.py
index c003cf6..a79ec0f 100755
--- a/scripts/tracetool.py
+++ b/scripts/tracetool.py
@@ -90,8 +90,8 @@
             arg_format = arg
 
         elif opt == "--list-backends":
-            backends = tracetool.backend.get_list()
-            out(", ".join([ b for b,_ in backends ]))
+            public_backends = tracetool.backend.get_list(only_public = True)
+            out(", ".join([ b for b,_ in public_backends ]))
             sys.exit(0)
         elif opt == "--check-backend":
             check_backend = True