XML reports (executable) covered lines and all lines (in <source>).
That also mean both uncovered lines and non-executable lines are absent.
As such, it seems impossible to only extract uncovered lines. What would you suggest to use?

/edit
(Actually used: xmlstarlet sel -t -m "//file/line[@count=0]" -v "../@name" -o " num:" -v "@num" -n clover-coverage.xml)