Currently the XML parser already allows the following syntax:
  <disk type='block' device='cdrom'>
    <source startupPolicy='optional'/>
    <target dev='hda' bus='ide'/>
    <address type='drive' controller='0' bus='0' target='0' unit='0'/>
  </disk>

But it if the dev value is NULL then it would not have the leading
"<source ", resulting in invalid XML.