dog: print new VID of current VDI when snapshot is created

Especially new VID of current VDI is useful information. So this patch
lets dog print both of the IDs.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>