migration: Check that migration is active before cancel it

Signed-off-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>

migration.c

@@ -133,9 +133,9 @@ int do_migrate_cancel(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     MigrationState *s = current_migration;
 
-    if (s)
+    if (s && s->get_status(s) == MIG_STATE_ACTIVE) {
         s->cancel(s);
-
+    }
     return 0;
 }