migration: Teach it about G_SOURCE_REMOVE

As this is defined on glib 2.32, add compatibility macros for older glibs.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
diff --git a/migration/exec.c b/migration/exec.c
index 08b599e..f3be1ba 100644
--- a/migration/exec.c
+++ b/migration/exec.c
@@ -49,7 +49,7 @@
 {
     migration_channel_process_incoming(ioc);
     object_unref(OBJECT(ioc));
-    return FALSE; /* unregister */
+    return G_SOURCE_REMOVE;
 }
 
 void exec_start_incoming_migration(const char *command, Error **errp)