add ploop fs driver type

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>