dog: allow vdi snapshot without snapshot tag

The commit a21bf27906b23448b92cca9943e1019105ffac2f makes
$ dog vdi snapshot <vdi>
fail if the <vdi> is an ordinal vdi because newly created VDIs have
snapid 0. This patch avoids the failure with checking the VDI is
snapshot or not.

Reviewed-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>