qemu/queue.h: do not access tqe_prev directly

Use the QTAILQ_IN_USE macro instead, it does the same thing but the next
patch will change it to a different definition.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

blockdev.c

@@ -4252,7 +4252,7 @@ void qmp_blockdev_del(const char *node_name, Error **errp)
         goto out;
     }
 
-    if (!bs->monitor_list.tqe_prev) {
+    if (!QTAILQ_IN_USE(bs, monitor_list)) {
         error_setg(errp, "Node %s is not owned by the monitor",
                    bs->node_name);
         goto out;