python: ensure QEMUQtestProtocol closes its socket

While QEMUQtestMachine closes the socket that was passed to
QEMUQtestProtocol, the python resource leak manager still
believes that the copy QEMUQtestProtocol holds is open. We
must explicitly call close to avoid this leak warnnig.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>