Fix clone issues

This commit fixes two issues at the time of clone:
* unnecessary directory is created when clone between overlayfs/aufs
* clone failed when the end of rootfs path is not "/rootfs"

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>