sheep, dog: check cluster is formatted or not during vdi creation

Current dog prints an odd error message in a case of vdi creation
before cluster formatting like below:
$ dog/dog vdi create test 16M
VDI size is larger than 1.0 MB bytes, please use '-y' to create a hyper volume with size up to 16 PB bytes or use '-z' to create larger object size volume

This patch revives previous behavior.

Reviewed-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>