ffmpeg: Add an option "qsv_device" to choose proper node for QSV child device (vaapi or dxva2)

Reason: For some cases, such as 2 or more graphics cards existing, the
default command line may fail because ffmpeg does not open the correct
device node:
    ffmpeg -hwaccel qsv -c:v h264_qsv -i test.264 -c:v h264_qsv out.264
Let user choose the proper one by running like below:
    ffmpeg -hwaccel qsv -qsv_device /dev/dri/renderD128 -c:v h264_qsv \
-i test.264 -c:v h264_qsv out.264

Signed-off-by: ChaoX A Liu <chaox.a.liu@gmail.com>
Signed-off-by: Huang, Zhengxu <zhengxu.maxwell@gmail.com>
Signed-off-by: Andrew, Zhang <huazh407@gmail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>