virtfs-proxy-helper: Make the helper_opts[] array const

Reduce a bit the memory footprint by making the helper_opts[]
array const.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200305010446.17029-4-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>