add ploop fs driver type
    
    Ploop is a pseudo device which makeit possible to access
    to an image in a file as a block device. Like loop devices,
    but with additional features, like snapshots, write tracker
    and without double-caching.
    
    It used in PCS for containers and in OpenVZ. You can manage
    ploop devices and images with ploop utility
    (http://git.openvz.org/?p=ploop).
    
    Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>