Fix ProcessLookupError on _checkAlive Qemu

Fix #308

gns3server/modules/qemu/qemu_vm.py

@@ -907,7 +907,10 @@ class QemuVM(BaseVM):
                         self._process.terminate()
                         yield from gns3server.utils.asyncio.wait_for_process_termination(self._process, timeout=3)
                 except asyncio.TimeoutError:
-                    self._process.kill()
+                    try:
+                        self._process.kill()
+                    except ProcessLookupError:
+                        pass
                     if self._process.returncode is None:
                         log.warn('QEMU VM "{}" PID={} is still running'.format(self._name, self._process.pid))
             self._process = None