ConfTool: check joystick before adding it

Some keyboards aren't compatible with SDL and are detected as joysticks. Kalehrl
found how to fix here:
https://code.google.com/p/mame4all-pi/issues/detail?id=7&can=1
Input: don't use SDL joystick index as player index

Since some SDL joysticks are ignored because they are in fact non compatible
keyboards, player index is not always the same as joystick index. We need to
find the player corresponding to joystick index.