lxc: add clone hook.

expose clone hook in api_clone, and in lxc-clone

The clone update hook is called while the container's bdev is mounted.
Information about the container is passed in through environment
variables LXC_ROOTFS_PATH, LXC_NAME, The LXC_ROOTFS_MOUNT, and
LXC_CONFIG_FILE.

args are passed in as "demo3 lxc clone hey there dude"

First argument is container name, second is always 'lxc', third
is the hook name (always clone), then come the arguments which
were passed to lxc-clone.  I.e. for the example above I did:

	sudo lxc-clone demo2 demo3 -- hey there dude

I personally would like to drop the first two arguments.  THe
name is available as $LXC_NAME, and the section argument ('lxc')
is meaningless.  However, doing so risks invalidating existing
hooks.

Soon analogous create and destroy hooks will be added as well.

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