add method volume_rollback_is_possible and redactor
Improve error handling
If snapshot is not reversible catch it before vm will lock and shutdown.
This is the case if zfs has an younger snapshot.
Signed-off-by: Wolfgang Link <w.link@proxmox.com>
Turned out it makes no sense to duplicated DirPlugin features. So I
also changed the name to make it less confusing. So we can only
create zvols inside a zfs pool with this plugin.
