qvm-tools/qvm-shutdown: do not kill already dead VM

When VM stop just when the timeout expires, qvm-shutdown should not try
to kill it - this would result in QubesException("VM already stopped!").

qvm-tools/qvm-shutdown

@@ -99,6 +99,7 @@ def main():
             for vm in vms_list:
                 if not vm.is_running():
                     vms_list.remove (vm)
+                    continue
                 if shutdown_counter > defaults["shutdown_counter_max"]:
                     # kill the VM
                     if options.verbose: