Eu usaria sed para essa tarefa:
sed -r -e 's/^<(.*spiral.*)>$/<!---->/'
Demo:
$ cat test.xml
<some line here>
<other spiral line here>
$ sed -i -r -e 's/^<(.*spiral.*)>$/<!---->/' test.xml
$ cat test.xml
<some line here>
<!--other spiral line here-->