Use SO_REUSEADDR before calling bind() where missing. Fixes #1289.

gns3server/controller/gns3vm/virtualbox_gns3_vm.py

@@ -191,6 +191,7 @@ class VirtualBoxGNS3VM(BaseGNS3VM):
         try:
             # get a random port on localhost
             with socket.socket() as s:
+                s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
                 s.bind((ip_address, 0))
                 api_port = s.getsockname()[1]
         except OSError as e: