Harden XML parsing

[jsegitz]

Orignal report: https://bugzilla.suse.com/show_bug.cgi?id=1143658

This module parse potentially untrusted XML. Without precautions this can result in DoS, network connections to machines in the internal network etc. For details please see the description in the bug

[jsegitz]

Also https://bugzilla.suse.com/show_bug.cgi?id=1143656

[adrianschroeter]

the network protection is handled in the network setup of build.opensuse.org server.
Not sure how any generic filtering of host names would look alike...

[jsegitz]

there are xml parsers that prevent the issues in the first place, lxml is unfortunately not really good here. So if we don't specifically need this we could move to a different parser that is better in this regard. The "real" protection comes from the network setup, but it would still be nice to use a better XML parser