QtSingleApplication: Fix race

If a starting instance locks the shared memory while another
instance that is destructed tries to lock, when the destructed
one tests for running peers, it doesn't detect the newly started
instance, since its QtLocalPeer object is not initialized until
after parsing the command-line arguments

Change-Id: I05aeb771ba67390f554bff7a80c475b3e4d37984
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>