qapi: fix build on glib < 2.28

The following commits:
    qapi: make string output visitor parse int list
    qapi: make string input visitor parse int list
break with glib < 2.28 since they use the
new g_list_free_full function.

Open-code that to fix build on old systems.

Cc: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index 1c0834a..8735b00 100644
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -316,13 +316,19 @@
     return &sov->visitor;
 }
 
+static void free_range(void *range, void *dummy)
+{
+    g_free(range);
+}
+
 void string_output_visitor_cleanup(StringOutputVisitor *sov)
 {
     if (sov->string) {
         g_string_free(sov->string, true);
     }
 
-    g_list_free_full(sov->ranges, g_free);
+    g_list_foreach(sov->ranges, free_range, NULL);
+    g_list_free(sov->ranges);
     g_free(sov);
 }