Introduce the qcrypto_tls_creds_check_endpoint() helper
to access QCryptoTLSCreds internal 'endpoint' field.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>