qapi: Introduce QAPI_LIST_APPEND

Similar to the existing QAPI_LIST_PREPEND, but designed for use where
we want to preserve insertion order.  Callers will be added in
upcoming patches.  Note the difference in signature: PREPEND takes
List*, APPEND takes List**.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210113221013.390592-4-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

include/qapi/util.h

@@ -37,4 +37,17 @@ int parse_qapi_name(const char *name, bool complete);
     (list) = _tmp; \
 } while (0)
 
+/*
+ * For any pointer to a GenericList @tail (usually the 'next' member of a
+ * list element), insert @element at the back and update the tail.
+ *
+ * Note that this macro evaluates @element exactly once, so it is safe
+ * to have side-effects with that argument.
+ */
+#define QAPI_LIST_APPEND(tail, element) do { \
+    *(tail) = g_malloc0(sizeof(**(tail))); \
+    (*(tail))->value = (element); \
+    (tail) = &(*(tail))->next; \
+} while (0)
+
 #endif