lxc-create: cleanup whenever exiting with error

Otherwise we leave bad containers sitting around and further confuse
things on retries.

Reported-by: Mukanyiligira Didacienne <siyana223@gmail.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>