tracetool: use Python 2.4-compatible __import__() arguments

In Python 2.5 keyword arguments were added to __import__().  Avoid using
them to achieve Python 2.4 compatibility.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: NLluís Vilanova <vilanova@ac.upc.edu>

scripts/tracetool/__init__.py

@@ -204,7 +204,7 @@ def try_import(mod_name, attr_name = None, attr_default = None):
     object or attribute value.
     """
     try:
-        module = __import__(mod_name, fromlist=["__package__"])
+        module = __import__(mod_name, globals(), locals(), ["__package__"])
         if attr_name is None:
             return True, module
         return True, getattr(module, str(attr_name), attr_default)