webadmin: PoolModelBehavior fix disk availability

* Removed a redundant call to setIsDisksAvailable,
  'setIsDisksAvailable' is already set correctly on InitDisks
  (causes an NPE in DisksAllocationModel -> sortDisks).
* UpdateIsDisksAvailable -> added the same logic
  as in NewVmModelBehavior.

Change-Id: I888147a7b61c49de7eb4ba9426f9e933113cb04b
Bug-Url: https://bugzilla.redhat.com/928410
Signed-off-by: Daniel Erez <derez@redhat.com>