tests: fix QED disk test in xlconfigtest

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.