vzctl snapshot-list: check if snapshot is supported

Rather than presenting empty snapshot list, bail out with an error,
or crash, check that
1 the container is ploop-based
2 ploop is supported

Crash would happen if ploop library is not avaialble, because
vzctl_env_snapshot_list() now calls ploop functions.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
3 files changed