userportal,webadmin: keep proper USB support selection

There were 2 problems:
1: the UnitVmModel.initUsbPolicy() did set the USB policy always
   to "disabled".
   Fixed by making sure that if the previously selected still applys than
   select it

2: The UnitVmModel.updateDisplayProtocol() have delegated the selection of the
   old display protocol to behavior.postDisplayTypeItemChanged which is too
   late, because meanwhile the VNC has been selected by
   getDisplayProtocol().setItems() which fired an appropriate event which in
   turn changed the USB support to "disabled"
   Fixed by removing the logic from VmModelBehaviorBase and moved it into the
   updateDisplayProtocol() to make sure the event is fired only when needed.

Change-Id: I7e1b1048784e9d0c289fab13b4d8b9b80afd6cc9
Bug-Url: https://bugzilla.redhat.com/966415
Bug-Url: https://bugzilla.redhat.com/1080144
Bug-Url: https://bugzilla.redhat.com/1147827
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>