Remove lxml deprecated methods
Remove lxml deprecated methods

All our supported runtimes [1] are compatible with the recommended
alternatives.

The `getchildren` method is deprecated [2] since lxml 2.0 and will be removed
in future versions, these changes switch usages to `list(elem)` as
recommended in the official documentation [2].

[1] https://governance.openstack.org/tc/reference/runtimes/victoria.html#python-runtimes-for-train
[2] https://lxml.de/api/lxml.etree._Element-class.html#getchildren

Change-Id: Ida46f789d1b751d617b3b06e8a192a5031452567

openstack/cinder,mahak/cinder,openstack/cinder,mahak/cinder