qemu-options.hx: correct spice options streaming-video default document value to 'off'

since f1d3e586, the code had changed the default value to 'off', so this patch
make document and code are consistent.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Message-id: 1470024419-10886-1-git-send-email-lizhijian@cn.fujitsu.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>