util: improve the sysinfo element XML format
If the <sysinfo type='smbios'...> ends up not formatting any sub-elements,
then rather than formatting as:
<sysinfo type='smbios'>
</sysinfo>
Just format it more cleanly as:
<sysinfo type='smbios'/>
Signed-off-by: Luyao Huang <lhuang@redhat.com>