tests/functional: add test for temporary object lost

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: use default copy number when vdi state is not available

Currently, screen_object_list() drops an object from the recovery list
when its copy number is not available.  This can happen, e.g., when we
lost all the inode replicas temporary.  To fix the problem, this patch
makes get_vdi_copy_number() return the default copy number when sheep
doesn't have the vdi state.

This patch is necessary to pass test/functional/074.

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