QmlDebugger: Close connection already in shutdownInferior

Doing it in shutdownEngine was too late for the mixed debugger
case: The client has already exited in this case, leading to
error messages on the (system) console.

Change-Id: Id31214cb7a1bc9530c1da78b872531bc79894f3b
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>