Refactor aux port allocation

This move the allocation of aux port to the base vm.
Also now the free of console port during the close is in the
base VM.

An aux port is allocated to the docker container but not used
for the moment.

Ref https://github.com/GNS3/gns3-gui/issues/1039

gns3server/modules/virtualbox/virtualbox_vm.py

@@ -60,7 +60,6 @@ class VirtualBoxVM(BaseVM):
         self._system_properties = {}
         self._telnet_server_thread = None
         self._serial_pipe = None
-        self._closed = False
 
         # VirtualBox settings
         self._adapters = adapters
@@ -344,14 +343,8 @@ class VirtualBoxVM(BaseVM):
         Closes this VirtualBox VM.
         """
 
-        if self._closed:
-            # VM is already closed
-            return
-
-        log.debug("VirtualBox VM '{name}' [{id}] is closing".format(name=self.name, id=self.id))
-        if self._console:
-            self._manager.port_manager.release_tcp_port(self._console, self._project)
-            self._console = None
+        if not (yield from super().close()):
+            return False
 
         for adapter in self._ethernet_adapters.values():
             if adapter is not None:
@@ -404,9 +397,6 @@ class VirtualBoxVM(BaseVM):
                                                                                                               id=self.id,
                                                                                                               error=e.strerror))
 
-        log.info("VirtualBox VM '{name}' [{id}] closed".format(name=self.name, id=self.id))
-        self._closed = True
-
     @property
     def headless(self):
         """