dog/vdi: stop preallocation for no share option in vdi cloning

Previous patch add a -n (--no-share) option in dog vdi clone.
Preallocation is also enabled by this option. However, it is
unnecessary to preallocate data object. In fact, we only need to
copy parent's data objects into its own.

This patch fix this problem to carry out thin-provision.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>