ploop.ovzl

Add 'ploop list' command

Scan /sys/block/ploop* and list all configured ploop devices in the
following format:

  device base_delta cookie

Ignore all errors, added for debug purpoes

[kir@openvz.org: patch merging, rm ploop_err(), export read_line()]

Reference: ploop@795206, ploop@795341, ploop@795364.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>