virttest.env_process: Include option to select encode video format.

Include support for option `encode_video_format` to output video with selected encoding format. Valid values are: `webm`, `ogg` or `automatic`.

Note that if this option is not setted, automatic detection will occurs: try for webm or fallback to ogg.

Signed-off-by: Ruda Moura <rmoura@redhat.com>