Allow instance disks to be added with --no-wait-for-sync

Allow instance disks to be added with --no-wait-for-sync

The patch 3c260845147c6dad35e37c03ba9a7556814f3f3f fixed a bug where
adding a new disk to an instance with deactivated disks activated the
disk. However, it also introduced some erroneous behaviour, preventing
disks from being added to online instances with --no-wait-for-sync.

In line with the original meaning of the patch, this patch modifies the
check to disallow adding disks to shutdown instances with
--no-wait-for-sync, and allow doing so for online instances.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>