Short answer: because python-lxml approach to work with xml namespaces is quite a pain.

Long answer:

we need to build element with xml namespace like 

```
+            <qemu:commandline xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">
+               <qemu:arg value="-global"/>
+               <qemu:arg value="virtio-blk-pci.scsi=off"/>
+               <qemu:arg value="-global"/>
+               <qemu:arg value="virtio-blk-pci.x-data-plane=on"/>
+            </qemu:commandline>
```

to enable dataplane feature with additional qemu commands and, well, merge it with root.

There is more straightforward register_namespace() function in lxml 3.x but unfortunately it does not compatible with 2.x version from EPEL.

Does it answer your question ?