webadmin: avoid null cluster on VmModel update vscsi

Avoid null cluster on VmModelBehaviorBase -> updateVirtioScsiEnabled.

Change-Id: Ie002f8d84b7e6449b179703dd471f674ca0e3f05
Related-To-Bug-Url: https://bugzilla.redhat.com/1038613
Signed-off-by: Daniel Erez <derez@redhat.com>