clone api: support a different size

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
implement overlayfs

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
make overlayfs-of-overlafys clones work

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>