Comment 2 for bug 1757928

I guess we've got this issue since this change: https://review.openstack.org/#/c/544809/2/os_xenapi/dom0/etc/xapi.d/plugins/glance.py@77

which change the module to making the code compatible with both py2 and py3.

We need change the UT script to use import the same modules, so that the mocks can take effective exactly.