crypto: reduce duplication in handling TLS priority strings

The logic for setting the TLS priority string on a session object has a
significant amount of logic duplication across the different credential
types. By recording the extra priority string suffix against the
credential class, we can introduce a common method for building the
priority string. The TLS session can now set the priority string without
caring about the credential type.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

include/crypto/tlscreds.h

@@ -47,6 +47,7 @@ typedef bool (*CryptoTLSCredsReload)(QCryptoTLSCreds *, Error **);
 struct QCryptoTLSCredsClass {
     ObjectClass parent_class;
     CryptoTLSCredsReload reload;
+    const char *prioritySuffix;
 };
 
 /**
@@ -64,4 +65,16 @@ bool qcrypto_tls_creds_check_endpoint(QCryptoTLSCreds *creds,
                                       QCryptoTLSCredsEndpoint endpoint,
                                       Error **errp);
 
+
+/**
+ * qcrypto_tls_creds_get_priority:
+ * @creds: pointer to a TLS credentials object
+ *
+ * Get the TLS credentials priority string. The caller
+ * must free the returned string when no longer required.
+ *
+ * Returns: a non-NULL priority string
+ */
+char *qcrypto_tls_creds_get_priority(QCryptoTLSCreds *creds);
+
 #endif /* QCRYPTO_TLSCREDS_H */