We allow a seclabel to be specified in the <source> element
of a chardev:
<serial type='file'>
<source path='/tmp/serial.file'>
<seclabel model='dac' relabel='no'/>
</source>
</serial>
But we format it outside the source:
<serial type='file'>
<source path='/tmp/serial.file'/>
<target port='0'/>
<seclabel model='dac' relabel='no'/>
</serial>
Move the formatting inside the source to fix this to make the
seclabel persistent across XML format->parse.
Introduced by commit f8b08d0 'Add <seclabel> to character devices.'
