clone_paths: use 'rootfs' for destination directory

clone_paths: use 'rootfs' for destination directory

We were trying to be smart and use whatever the last part of
the container's rootfs path was.  However for block devices
that doesn't make much sense.  I.e. if lxc.rootfs = /dev/md-1,
chances are that /var/lib/lxc/c1/md-1 does not exist.

So always use the $lxcpath/$lxcname/rootfs, and if it does
not exist, try to create it.

With this, 'lxc-clone -s -o c1 -n c2' where c1 has an lvm backend
is fixed.  See https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1414771

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>