In preparation of removing the legacy Xen driver, move the
xml2sexpr tests from WITH_XEN to WITH_LIBXL. Even though the
legacy driver will be removed, we'll want to maintain the ability
to convert sexpr <-> XML.

There is some test file fallout due to differences in handling
of default values between xend and libxl.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>