migration: Clean up around tls_creds, tls_hostname

Optional MigrationParameters members tls_creds and tls_hostname can't
actually be absent outside qmp_migrate_set_parameters() since commit
4af245dc (v2.9.0).

Note that commit 4af245dc reverted the part of commit de63ab61 (v2.8.0)
that made tls_creds and tls_hostname absent instead of "" in the value
of query-migrate-parameters, even though commit de63ab61 called that a
mistake.  What a mess.

Drop the redundant tests for presence, and update documentation.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

hmp.c

@@ -313,12 +313,14 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)
         monitor_printf(mon, "%s: %" PRId64 "\n",
             MigrationParameter_lookup[MIGRATION_PARAMETER_CPU_THROTTLE_INCREMENT],
             params->cpu_throttle_increment);
+        assert(params->has_tls_creds);
         monitor_printf(mon, "%s: '%s'\n",
             MigrationParameter_lookup[MIGRATION_PARAMETER_TLS_CREDS],
-            params->has_tls_creds ? params->tls_creds : "");
+            params->tls_creds);
+        assert(params->has_tls_hostname);
         monitor_printf(mon, "%s: '%s'\n",
             MigrationParameter_lookup[MIGRATION_PARAMETER_TLS_HOSTNAME],
-            params->has_tls_hostname ? params->tls_hostname : "");
+            params->tls_hostname);
         assert(params->has_max_bandwidth);
         monitor_printf(mon, "%s: %" PRId64 " bytes/second\n",
             MigrationParameter_lookup[MIGRATION_PARAMETER_MAX_BANDWIDTH],

migration/migration.c

@@ -444,9 +444,9 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
     params->cpu_throttle_initial = s->parameters.cpu_throttle_initial;
     params->has_cpu_throttle_increment = true;
     params->cpu_throttle_increment = s->parameters.cpu_throttle_increment;
-    params->has_tls_creds = !!s->parameters.tls_creds;
+    params->has_tls_creds = true;
     params->tls_creds = g_strdup(s->parameters.tls_creds);
-    params->has_tls_hostname = !!s->parameters.tls_hostname;
+    params->has_tls_hostname = true;
     params->tls_hostname = g_strdup(s->parameters.tls_hostname);
     params->has_max_bandwidth = true;
     params->max_bandwidth = s->parameters.max_bandwidth;

qapi-schema.json

@@ -1054,9 +1054,7 @@
 # @MigrationParameters:
 #
 # Optional members can be omitted on input ('migrate-set-parameters')
-# but most members will always be present on output
-# ('query-migrate-parameters'), with the exception of tls-creds and
-# tls-hostname.
+# but members will always be present on output.
 #
 # @compress-level: compression level
 #
@@ -1077,10 +1075,10 @@
 #             channel. On the outgoing side of the migration, the credentials
 #             must be for a 'client' endpoint, while for the incoming side the
 #             credentials must be for a 'server' endpoint. Setting this
-#             will enable TLS for all migrations. The default is unset,
-#             resulting in unsecured migration at the QEMU level. (Since 2.7)
+#             to a non-empty string enables TLS for all migrations.
 #             An empty string means that QEMU will use plain text mode for
-#             migration, rather than TLS (Since 2.9)
+#             migration, rather than TLS (Since 2.7)
+#             Note: 2.8 reports this by omitting tls-creds instead.
 #
 # @tls-hostname: hostname of the target host for the migration. This
 #                is required when using x509 based TLS credentials and the
@@ -1090,6 +1088,7 @@
 #                certificate identity can be validated. (Since 2.7)
 #                An empty string means that QEMU will use the hostname
 #                associated with the migration URI, if any. (Since 2.9)
+#                Note: 2.8 reports this by omitting tls-hostname instead.
 #
 # @max-bandwidth: to set maximum speed for migration. maximum speed in
 #                 bytes per second. (Since 2.8)