Use new virtual IO api

Use new virtual IO api

	* src/media/media-ffmpeg.c: Remove the no-longer used
	media_ffmpeg_vio_open, media_ffmpeg_vio_close,
	media_ffmpeg_vio_write.
	(media_ffmpeg_vio_read): Use the new opaque pointer interface of
	AVIOContext, which points to our media_data struct directly.
	(media_ffmpeg_vio_seek): Ditto.
	(media_ffmpeg_open_data): Use the new AVIOContext and allocate one
	pointing to media_ffmpeg_vio_read, media_ffmpeg_vio_seek and with
	opaque data as our media_data.  Use the context to open the input.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>