tests/func: add a test for massive clone

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

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: clear child vdi for deletion of clone

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

Signed-off-by: Liu Yuan <namei.unix@gmail.com>