qemu: domain: Add XML namespace code for overriding device config

qemu: domain: Add XML namespace code for overriding device config

Implement the XML parser and formatter for overriding of device
properties such as:

  <qemu:override>
    <qemu:device alias='ua-disk'>
      <qemu:frontend>
        <qemu:property name='prop1' type='string' value='propval1'/>
        <qemu:property name='prop2' type='signed' value='-321'/>
        <qemu:property name='prop3' type='unsigned' value='123'/>
        <qemu:property name='prop4' type='bool' value='true'/>
        <qemu:property name='prop5' type='bool' value='false'/>
        <qemu:property name='prop6' type='bool' value='false'/>
        <qemu:property name='prop6' type='remove'/>
      </qemu:frontend>
    </qemu:device>
  </qemu:override>

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>