don't virtual a class that shouldn't be inheritable, use final instead

virtual marks a method for inclusion on a v-table, but we don't want
the methods of classes that won't be inheritable to have a v-table,
we want, however, to issue a warning if somebody try to inherit
that specific class and reimplement those methods.

Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>