sheep: remove finish_join()

Just call sockfd_cache_add_group directly.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: return default copy number as formated

We should return cinfo.nr_copies, which is the number as formated, for
get_vdi_copy_number() if copy number isn't found during init.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>