docs: Document bhyve's vgaconf support
    
     - Add a news entry
     - Update driver's page with information about the new
       vgaconf attribute and provide usage example; while here,
       fix a grammar mistake
    
    Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>