collie: don't create VDI when there are not enough nodes

If we have less nodes than required copies, just error return.

Reported-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>