dog: fix improper typed 'ret' in 'vdi create'

'ret' is unsigned then never get < 0 even in error case, so for a wrong size, we
will proceed to create an invalid vdi.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>

Conflicts:
	dog/vdi.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
dog: remove needless checking of vdi creation

The removed check can be done in sheep side.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>