use btrfs snapshot feature to restore snapshots
use btrfs snapshot feature to restore snapshots

fixes #131

changes since v1;
* uses btrfs snapshot feature only if src and dest are on same fs

Signed-off-by: S.Çağlar Onur <acf5ae661bb0a9f738c88a741b1d35ac69ab5408@10ur.org>
Acked-by: Serge E. Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

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