Ensure we can't connect to occupy port

Ref https://github.com/GNS3/gns3-gui/issues/1759

gns3server/controller/link.py

@@ -58,6 +58,9 @@ class Link:
         """
 
         port = node.get_port(adapter_number, port_number)
+        if port.link is not None:
+            raise aiohttp.web.HTTPConflict(text="Port is already used")
+
         self._link_type = port.link_type
 
         for other_node in self._nodes:
@@ -86,6 +89,7 @@ class Link:
             "node": node,
             "adapter_number": adapter_number,
             "port_number": port_number,
+            "port": port,
             "label": label
         })
 
@@ -93,6 +97,7 @@ class Link:
             yield from self.create()
             for n in self._nodes:
                 n["node"].add_link(self)
+                n["port"].link = self
             self._created = True
             self._project.controller.notification.emit("link.created", self.__json__())
 
@@ -123,8 +128,11 @@ class Link:
         """
         Delete the link
         """
-        for port in self._nodes:
-            port["node"].remove_link(self)
+        for n in self._nodes:
+            # It could be different of self if we rollback an already existing link
+            if n["port"].link == self:
+                n["port"].link = None
+                n["node"].remove_link(self)
 
     @asyncio.coroutine
     def start_capture(self, data_link_type="DLT_EN10MB", capture_file_name=None):