Raise error if IOU image is not configured

Fix #757

gns3server/compute/iou/iou_vm.py

@@ -169,6 +169,8 @@ class IOUVM(BaseNode):
         Checks if IOUYAP executable is available and if image is accessible.
         """
 
+        if not self._path:
+            raise IOUError("IOU image is not configured")
         if not os.path.isfile(self._path) or not os.path.exists(self._path):
             if os.path.islink(self._path):
                 raise IOUError("IOU image '{}' linked to '{}' is not accessible".format(self._path, os.path.realpath(self._path)))