Catch ProcessLookupError in Qemu VM.

gns3server/modules/qemu/qemu_vm.py

@@ -611,6 +611,8 @@ class QemuVM(BaseVM):
                     log.error("Cannot stop the Qemu process: {}".format(e))
                 if self._process.returncode is None:
                     log.warn('QEMU VM "{}" with PID={} is still running'.format(self._name, self._process.pid))
+            except ProcessLookupError:
+                pass
         self._process = None
         self._started = False
         self._stop_cpulimit()