fork off a task to delete ovs ports when done

fork off a task to delete ovs ports when done

The new task waits until the container is STOPPED, then asks
openvswitch to delete the port.

This requires two new arguements to be sent to lxc-user-nic.
Since lxc-user-nic ships with lxc, this shouldn't be a problem.

Finally when calling lxc-user-nic, use execlp insteac of execvp
to preserve lxcpath's const-ness.  Technically we are
guaranteed that execvp won't change the args, but it's worth
it to silence the warnings (and not hide real errors).

With this patch, container nics are cleaned up from openvswitch
bridges on shutdown.

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