libvirt: lxml behavior breaks version check.
libvirt: lxml behavior breaks version check.

lxml doesn't roundtrip empty strings, meaning if you serialize
and element with value '', when you derserialize it, the value will be
`None`.

This broke the version_string_with_package check because None and ''
will not compare as equivalent.

The solution is to coerce the None value back to an empty string before
running the check.

Change-Id: Ia3a8ae68f12a4645057d243fdee59197df40e8d3

n0ano/gantt,n0ano/gantt