webadmin: Fix ClassCastException in StorageListModel

Due to a change introduced in patch 27578 the GetHostsForStorageOperation
query returns a LinkedList instead of an ArrayList, StorageListModel
wrongly casted the result explicitely to ArrayList which resulted in a
ClassCastException making it impossible to remove a storage domain

Change-Id: I8d16d99b51dbc55be1805c832f79bdf83c1d6764
Signed-off-by: Tal Nisan <tnisan@redhat.com>