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

gns3server/compute/base_node.py

@@ -514,7 +514,7 @@ class BaseNode:
         try:
             yield from self._ubridge_hypervisor.send(command)
         except UbridgeError as e:
-            raise UbridgeError("{}: {}".format(e, self._ubridge_hypervisor.read_stdout()))
+            raise UbridgeError("Error while sending command '{}': {}: {}".format(command, e, self._ubridge_hypervisor.read_stdout()))
 
     @locked_coroutine
     def _start_ubridge(self):

gns3server/compute/qemu/qemu_vm.py

@@ -894,6 +894,7 @@ class QemuVM(BaseNode):
                         af, socktype, proto, _, sa = res
                         # let the OS find an unused port for the Qemu monitor
                         with socket.socket(af, socktype, proto) as sock:
+                            sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
                             sock.bind(sa)
                             self._monitor = sock.getsockname()[1]
                 except OSError as e: