tests: Fix libxlxml2domconfigtest with latest xen

shadow_memkb is populated from a libxl API call, and the value can
change. For example:
https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=2c992810854a15b41be920519ce83a4a328d5168

Mock libxl_get_required_shadow_memory to give consistent output

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>

tests/libxlxml2domconfigdata/fullvirt-cpuid-legacy-nest.json

@@ -11,7 +11,7 @@
         ],
         "max_memkb": 592896,
         "target_memkb": 403456,
-        "shadow_memkb": 5656,
+        "shadow_memkb": 1234,
         "cpuid": [
             {
                 "leaf": 1,