Support for .xml testvector description file

[dsilhavy]

Description

From Jon Piesing:

All the media segments are (typically?) in a single file for ease of management. A trivial .xml file defines byte offsets and lengths within that single file so segments can be extracted into individual files when installed on a web server. Adding support for this file to the validator is an HbbTV-specific improvement as this would allow running the validator against the HbbTV SVN repository without unpacking the media segments, e.g. whenever anyone commits to the repository.