Re-fix the ConfigureWindow request padding issue.

Re-fix the ConfigureWindow request padding issue.

We rely on the fact that the valueparam field is treated as a list,
and, like any other list, the Python code will check if the length
field is previously defined in the structure before adding a new one.
This allows us to insert the necessary 2-byte padding.

Signed-off-by: Eamon Walsh <ewalsh@tycho.nsa.gov>