webadmin: NPE on new VM dialog

The problem was that the VDSGroup.requiredRngSources was marked as final thus
the GWT deserializer inited it to null. Consequently, the
VmModelBehaviorBase.setRngAvailability failed on NPE.

Change-Id: I0eac061be4370cdef57c5c7b466f7a8ec5bcda01
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>