When using QEMU's 9pfs the target "dir" element is not necessarily an
absolute path but merely an arbitrary identifier. So validation in that
case currently fails with the misleading

   $ virt-xml-validate /tmp/test.xml
   Relax-NG validity error : Extra element devices in interleave
   /tmp/test.xml:24: element devices: Relax-NG validity error : Element domain failed to validate content
   /tmp/test.xml fails to validate