collie: confirm when rolling back to the snapshot

'collie vdi rollback' discards any changes made since the previous
snapshot was taken, so it is safe to confirm it to users before doing
the operation.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
collie: restore snapshot only when stdin is pipe

We don't expect that stdin is console when doing the 'vdi restore'
command.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>