implement loopback backing store

Create a loopfile backed container by doing:

	lxc-create -B loop -t template -n name

or

	lxc-clone -B loop -o dir1 -n loop1

The rootfs in the configuration file will be

	loop:/var/lib/lxc/loop1/rootdev

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