Originally, inside <cputune/cachetune>, it requires the <cache> element to
be in the position before <monitor>, and following configuration is not
permitted by schema, but it is better to let it be valid.

  <cputune>
    <cachetune vcpus='0-1'>
      <monitor level='3' vcpus='0-1'/>
            ^
            |__ Not permitted originally because it is in the place
                before <cache> element.

      <cache id='0' level='3' type='both' size='3' unit='MiB'/>
      <cache id='1' level='3' type='both' size='3' unit='MiB'/>
    </cachetune>
    ...
  </cputune>

And, let schema do more strict check by identifying following configuration to
be invalid, due to <cachetune> should contain at least one <cache> or <monitor>
element.

  <cputune>
    <cachetune vcpus='0-1'>
        ^
        |__ a <cachetune> SHOULD contain at least one <cache> or <monitor>

    </cachetune>
    ...
  </cputune>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NHuaqiang <huaqiang.wang@intel.com>