1. Some templates copy the cached pristine rootfs using 'cp a b' where b is
$lxc_path/$name/rootfs.  That doesn't do the right thing if rootfs already
exists, as it will when it is an lvm or other mount.  So switch to
'rsync a/ b/'.  (cp can be made to work too of course).

2. Update lxc-create to support backing stores.  For now only lvm is
implemented.
Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>