crypto: mandate a hostname when checking x509 creds on a client

Currently the TLS session object assumes that the caller will always
provide a hostname when using x509 creds on a client endpoint. This
relies on the caller to detect and report an error if the user has
configured QEMU with x509 credentials on a UNIX socket. The migration
code has such a check, but it is too broad, reporting an error when
the user has configured QEMU with PSK credentials on a UNIX socket,
where hostnames are irrelevant.

Putting the check into the TLS session object credentials validation
code ensures we report errors in only the scenario that matters.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220304193610.3293146-2-berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

crypto/tlssession.c

@@ -373,6 +373,12 @@ qcrypto_tls_session_check_certificate(QCryptoTLSSession *session,
                                session->hostname);
                     goto error;
                 }
+            } else {
+                if (session->creds->endpoint ==
+                    QCRYPTO_TLS_CREDS_ENDPOINT_CLIENT) {
+                    error_setg(errp, "No hostname for certificate validation");
+                    goto error;
+                }
             }
         }