drm/xe: skip Kunit tests requiring real hardware when running on UML

Some tests are meant to run only on real hardware. Skip those,
if no device was found.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>