tests: fix QED disk test in xlconfigtest
    
    When LIBXL_HAVE_QED is defined, xlconfigtest fails
    
     9) Xen XL-2-XML Format disk-qed  ... command line: config parsing error
     in disk specification: no vdev specified in
     `target=/var/lib/libvirt/images/XenGuest2,format=qed,backendtype=qdisk,vdev=hda,access=rw'
    FAILED
    
    As per the xl-disk-configuration(5) man page, target= must come
    last in the disk specification when specified by name:
    
    When this parameter is specified by name, ie with the target=
    syntax in the configuration file, it consumes the whole rest of the
    DISKSPEC including trailing whitespaces.  Therefore in that case
    it must come last.
    
    Change tests/xlconfigdata/test-disk-qed.cfg to adhere to this
    restriction.