monitor: replace the magic number 255 with MAX_QUEUE_NUM This patch replace the magic number 255, and increase it to MAX_QUEUE_NUM which is maximum number of queues supported by a nic. Cc: Luiz Capitulino <lcapitulino@redhat.com> Signed-off-by: Jason Wang <jasowang@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>