Fix QDBusAbstractInterface::isValid() for peer connections

Do not attempt to lookup the service owner on peer connections (it will
fail).
Make QDBusAbstractInterface::isValid() return a sensible result on peer
connections, instead of always returning false.

Task-number: QTBUG-32374
Change-Id: I1b02feaffb3b255188f8d63306f89f5034a32f22
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>