Avoid consuming 100% of CPU when stopping docker container

Fix #427

gns3server/modules/docker/docker_vm.py

@@ -299,6 +299,11 @@ class DockerVM(BaseVM):
         """Stops this Docker container."""
 
         try:
+            if self._telnet_server:
+                self._telnet_server.close()
+                yield from self._telnet_server.wait_closed()
+                self._telnet_server = None
+
             if self._ubridge_hypervisor and self._ubridge_hypervisor.is_running():
                 yield from self._ubridge_hypervisor.stop()
 
@@ -306,10 +311,6 @@ class DockerVM(BaseVM):
             if state == "paused":
                 yield from self.unpause()
 
-            if self._telnet_server:
-                self._telnet_server.close()
-                self._telnet_server = None
-
             # t=5 number of seconds to wait before killing the container
             try:
                 yield from self.manager.query("POST", "containers/{}/stop".format(self._cid), params={"t": 5})