LUExportInstance: Accept instance already shut down

To remove the instance after an export it needs to be stopped. This can
be achived using the parameter “shutdown”, or by explicitly shutting
down the instance before exporting. The latter would still require the
“shutdown” parameter to be set. To make it more intuitive, this
requirement is changed with this patch. Instances already stopped are
accepted for automatic removal.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>