trace: avoid Python 2.5 all() in tracetool

Red Hat Enterprise Linux 5 ships Python 2.4.3.  The all() function was
added in Python 2.5 so we cannot use it.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NBenoît Canet <benoit.canet@nodalink.com>

scripts/tracetool/backend/__init__.py

@@ -102,7 +102,8 @@ class Wrapper:
     def __init__(self, backends, format):
         self._backends = [backend.replace("-", "_") for backend in backends]
         self._format = format.replace("-", "_")
-        assert all(exists(backend) for backend in self._backends)
+        for backend in self._backends:
+            assert exists(backend)
         assert tracetool.format.exists(self._format)
 
     def _run_function(self, name, *args, **kwargs):