Allow running instances to be put online

If an instance is running (eg. ERROR_up) and at the same time offline,
there's no way to either shut it down or reonline it. This allows
onlining it. Offlining is still disabled for running instances.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>