scripts/ci/setup: Fix libxen requirements

scripts/ci/setup: Fix libxen requirements

XEN hypervisor is only available in ARM and x86, but the yaml only
checked if the architecture is different from s390x, changed it to
a more accurate test.
Tested this change on a Ubuntu 20.04 ppc64le.

Signed-off-by: Lucas Mateus Castro (alqotel) <lucas.araujo@eldorado.org.br>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220922135516.33627-3-lucas.araujo@eldorado.org.br>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-3-alex.bennee@linaro.org>