Disabling a XTest device followed by an XTest API call crashes the server.
This could be fixed elsewhere but disabled devices must not send events
anyway. The use-case for disabled XTest devices is somewhat limited, so
simply disallow disabling the devices.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>