Fix some tests on travis

gns3server/compute/virtualbox/virtualbox_vm.py

@@ -209,7 +209,7 @@ class VirtualBoxVM(BaseNode):
         if os.path.exists(self._linked_vbox_file()):
             tree = ET.parse(self._linked_vbox_file())
             machine = tree.getroot().find("{http://www.virtualbox.org/}Machine")
-            if machine:
+            if machine is not None:
                 machine.set("uuid", "{" + self.id + "}")
                 tree.write(self._linked_vbox_file())