clone: clear the rootfs out of unexpanded config

clone: clear the rootfs out of unexpanded config

Closes #694

When we start cloning container c1 to c2, we first save c1's
configuration in c2's as a starting point.  We long ago cleared
out the lxc.rootfs entry before saving it, so that if we are
killed before we update the rootfs, c2's rootfs doesn't point
to c1's.  Because then lxc-destroy -n c2 would delete c1's rootfs.

But when we introduced the unexpanded_config, we didn't update
this code to clear the rootfs out of the unexpanded_config, which
is what now actually gets saved in write_config().

Do so.

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