tests/func: add a test for massive clone

Master can't pass it and following patch fixes it.

Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: clear child vdi for deletion

This will fix a bug that after deletion of clones, we fail to create clones
with the snapshot that we previousy cloned.

Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/vdi: fix vdi snapshot deletion

This fixes tests/func/044,046 failures and add some comment.

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