apibuild.py: fix TypeError raised in except clause

apibuild.py: fix TypeError raised in except clause

When an exception happened inside the try clause in serialize_function,
a new exception was raised in the except clause subsequently:

Traceback (most recent call last):
  File "./apibuild.py", line 2529, in <module>
    rebuild("libvirt")
  File "./apibuild.py", line 2513, in rebuild
    builder.serialize()
  File "./apibuild.py", line 2467, in serialize
    self.serialize_function(output, function)
  File "./apibuild.py", line 2208, in serialize_function
    self.warning("Failed to save function %s info: " % name, `id.info`)
TypeError: warning() takes exactly 2 arguments (3 given)

Use the correct number of arguments for self.warning and print the
original exception to stderr.