Also, use qint64 instead of quint64, as this is what Qt uses.
Keep 0 for invalid PIDs, even if arguably -1 would be better,
but Qt uses 0, too.
Don't move 'toString', instead inline it into the only
caller, keeping translations intact.
Task-number: QTCREATORBUG-17596
Change-Id: Ie9411ea1d2031a5ab0a99bcb3ff48ee430afe254
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
