ui: dc/storage: allow to renenable iscsi storages again

ui: dc/storage: allow to renenable iscsi storages again

The dc/storage refactoring done in
commit f28ec3e4b3e6cca1f45630e75f9858c558680979
introduced a regression where an once deactivated iSCSI storage could
not be activated again through UI, as we did not called the parent
setValue function correctly, thus the Enable checkbox was always set
to true. A uncheck -> check cycle couldn't also be done as the dirty
changes check disabled the submit button in this case.

Enable/Disable through API or CLI still worked.

Reported-by: Bastian Sebode <b.sebode@linet-services.de>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>