webadmin: Fix ClassCastException in AsyncDataProvier, DataCenterGuideModel

Due to a change introduced in patch 31cd7918c9c5eb5c2d6ddf5526b366fdb2d0f4bc
queries that once returned an ArrayList are now returning LinkedList
which resulted in a ClassCastException making the data center guide
dialogs stuck in the process

Change-Id: I7007e30fbd579b1055f8bfd1da558b723965a6cd
Signed-off-by: Tal Nisan <tnisan@redhat.com>