qobject: Clean up QLIST_FOREACH_ENTRY()

QLIST_FOREACH_ENTRY() traverses a tail queue manually.  Use
QTAILQ_FIRST() and QTAILQ_NEXT() instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200415083048.14339-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qapi/qmp/qlist.h

@@ -34,10 +34,10 @@ void qlist_append_int(QList *qlist, int64_t value);
 void qlist_append_null(QList *qlist);
 void qlist_append_str(QList *qlist, const char *value);
 
-#define QLIST_FOREACH_ENTRY(qlist, var)             \
-        for ((var) = ((qlist)->head.tqh_first);     \
-            (var);                                  \
-            (var) = ((var)->next.tqe_next))
+#define QLIST_FOREACH_ENTRY(qlist, var)                 \
+        for ((var) = QTAILQ_FIRST(&(qlist)->head);      \
+             (var);                                     \
+             (var) = QTAILQ_NEXT((var), next))
 
 static inline QObject *qlist_entry_obj(const QListEntry *entry)
 {