migration: Use cmpxchg correctly

cmpxchg returns the old value

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -509,7 +509,7 @@ void qmp_migrate_set_parameters(bool has_compress_level,
 
 static void migrate_set_state(MigrationState *s, int old_state, int new_state)
 {
-    if (atomic_cmpxchg(&s->state, old_state, new_state) == new_state) {
+    if (atomic_cmpxchg(&s->state, old_state, new_state) == old_state) {
         trace_migrate_set_state(new_state);
     }
 }