frontend: refactor in VmModelBehaviorBase

Replace usage of List+indexes with Map+enum values which are more
appropriate to store the timezone values per timezone type (windows
timezone, general timezone).

Change-Id: Ib7f12c4f8be93adfd0c1983a57c8e60c629ad331
Signed-off-by: Arik Hadas <ahadas@redhat.com>