Use correct signal name when disconnecting "NameOwnerChanged"

A disconnectSignal() call with a wrong signal name caused that hook
wasn't found and thus kept in QDBusConnectionPrivate::signalHooks
forever.

Change-Id: Id7cda225be7580529fc835b377636226abb229f9
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>