qapi: Introduce a first class 'null' type

I expect the 'null' type to be useful mostly for members of alternate
types.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/tests/test-qobject-input-visitor.c b/tests/test-qobject-input-visitor.c
index f98caf9..f451844 100644
--- a/tests/test-qobject-input-visitor.c
+++ b/tests/test-qobject-input-visitor.c
@@ -583,6 +583,11 @@
     g_assert_cmpint(tmp->u.e, ==, ENUM_ONE_VALUE1);
     qapi_free_UserDefAlternate(tmp);
 
+    v = visitor_input_test_init(data, "null");
+    visit_type_UserDefAlternate(v, NULL, &tmp, &error_abort);
+    g_assert_cmpint(tmp->type, ==, QTYPE_QNULL);
+    qapi_free_UserDefAlternate(tmp);
+
     v = visitor_input_test_init(data, "{'integer':1, 'string':'str', "
                                 "'enum1':'value1', 'boolean':true}");
     visit_type_UserDefAlternate(v, NULL, &tmp, &error_abort);