Description of problem:
When an xref refers to a listitem running clean_ids will null the listitem id
and leave an empty xref target, resulting in invalid DocBook.

Version-Release number of selected component (if applicable):
All.

How reproducible:
100%

Steps to Reproduce:
1. Create a list containing a listitem with an id
2. Create an xref refereing to the above id
3. make clean_ids
  
Actual results:
... element xref: validity error : IDREF attribute endterm references an unknown
ID "..."

Expected results:
no error.

Additional info:
It is valid to have xrefs to listitems, since listitems do not have titles
xmlClean should not change listitem ids.