migration: activate TLS thread safety workaround

When either the postcopy or return path capabilities are
enabled, the migration code will use the primary channel
for bidirectional I/O.

If either of those capabilities are enabled, the migration
code needs to mark the channel as expecting concurrent I/O
in order to activate the thread safety workarounds for
GNUTLS bug 1717

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1937
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/qemu-devel/20250718150514.2635338-4-berrange@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/tls.c

@@ -90,6 +90,10 @@ void migration_tls_channel_process_incoming(MigrationState *s,
 
     trace_migration_tls_incoming_handshake_start();
     qio_channel_set_name(QIO_CHANNEL(tioc), "migration-tls-incoming");
+    if (migrate_postcopy_ram() || migrate_return_path()) {
+        qio_channel_set_feature(QIO_CHANNEL(tioc),
+                                QIO_CHANNEL_FEATURE_CONCURRENT_IO);
+    }
     qio_channel_tls_handshake(tioc,
                               migration_tls_incoming_handshake,
                               NULL,
@@ -149,6 +153,11 @@ void migration_tls_channel_connect(MigrationState *s,
     s->hostname = g_strdup(hostname);
     trace_migration_tls_outgoing_handshake_start(hostname);
     qio_channel_set_name(QIO_CHANNEL(tioc), "migration-tls-outgoing");
+
+    if (migrate_postcopy_ram() || migrate_return_path()) {
+        qio_channel_set_feature(QIO_CHANNEL(tioc),
+                                QIO_CHANNEL_FEATURE_CONCURRENT_IO);
+    }
     qio_channel_tls_handshake(tioc,
                               migration_tls_outgoing_handshake,
                               s,