virt: Cartesian config update v2. RFC

Features:
<ul>
     <li> virt: Adds possibility to exclude filter from config. </li>
     <li> virt: Adds possibility filter defaults variant from variants </li>
     <li> virt: Adds named variants to Cartesian config.  </li>
</ul>
Unittest:
<ul>
     <li> virt: Extends Cartesian config unittests for new features     </li>
</ul>Debug:
<ul>
     <li> virt: Make Cartesian config more verbose </li>
     <li> virt: Adds formal definition for Cartesian config </li>
</ul>