fftools/qsv: add extra_hw_frames support

Currently extra_hw_frames can't be applied to qsv since it
doesn't call function avcodec_get_hw_frames_parameters().

Give an option to fix ticket #7261 though it is not a perfect soultion
(allocate the minimum pool size internally and automatically).

Signed-off-by: Zhong Li <zhong.li@intel.com>