webadmin: Fix NPE during OS selection

This fixes some NPEs in the oSType_SelectedItemChanged() method of 3
behaviors associated with the UnitVmModel class. These were caused by
an implicit cast from Integer to int of the currently selected OS.

Change-Id: I94b2aef851431fad201c77eec97a74a0fe61396b
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>