tests: Fix the libxl xml2domconfig test

Commit a13b2905 missed an adjustment to a test that is only run when
building against xen <= 4.9, where LIBXL_HAVE_BUILDINFO_NESTED_HVM is
not defined. Adjust fullvirt-cpuid-legacy-nest test similar to the others.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>

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

@@ -27,6 +27,9 @@
             "acpi": "True",
             "nested_hvm": "False",
             "nographic": "True",
+            "vga": {
+                "kind": "none"
+            },
             "vnc": {
                 "enable": "False"
             },