qemu: Fix name-space handling

qemu: Fix name-space handling

The XML parser for the qemu specific extensions expects the qemu name-space
to be bound to the 'qemu' prefix. This is too strict, since the name of the
name-space-prefix is only meant as an internal lookup key. Only the associated
URI is relevant.
<domain>...
  <qemu:commandline xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">
  ...</qemu:commandline>
</domain>

<domain xmlns:ns0="http://libvirt.org/schemas/domain/qemu/1.0">...
  <ns0:commandline>
  ...</ns0:commandline>
</domain>

<domain xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">
  <qemu:commandline xmlns:qemu="urn:foo">
  ...</qemu:commandline>
</domain>

Remove the test for checking the name-space binding on the top-level <domain>
element. Registering the name-space with XPath is enough.

Signed-off-by: Philipp Hahn <hahn@univention.de>