conf: format only relevant attributes for graphics based on listen type
    
    This patch changes following output:
    
        ...
        <graphics type='vnc' port='-1' autoport='yes'>
          <listen type='none'/>
        </graphics>
        ...
    
    into this output:
    
        ...
        <graphics type='vnc'>
          <listen type='none'/>
        </graphics>
        ...
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>