Fix the timeout == 0 case in lxcapi_shutdown()

The timeout argument should be handled as follows:
   -1 => Wait forever
    0 => Don't wait
  > 0 => Wait for timeout seconds

Without this patch, the 0 case is mapped to -1.

Signed-off-by: Robert Vogelgesang <vogel@users.sourceforge.net>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>