tests: move xml2sexpr tests to WITH_LIBXL

tests: move xml2sexpr tests to WITH_LIBXL

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: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>