frontend: ClassCastException in VmSnapshotListModel fix

The change in AsyncDataProvider.GetCustomPropertiesList in ada6bac1 caused
ClassCastException. This patch fixes handling result of this method in
VmSnapshotListModel.

Change-Id: Ie23f1994d218742133a7d963173e416d8e94cfcc
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>