userportal,webadmin: Changing cluster of existing VM changes OS type

Fixed by selecting the already selected OS type if it is still applicable under
the context of the newly selected cluster.

Change-Id: Ifaf27934e200e69beeb0912db22f26525b85e9b1
Bug-Url: https://bugzilla.redhat.com/1080056
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>