core: scheduling: fix ClassCastException

Since the external scheduler balance command returns
Host id as LinkedList and the migrate command expects
ArrayList the exception occurs.

Change-Id: I011d1d01b6d2de1718a73f687ed6d181ccadb135
Bug-Url: https://bugzilla.redhat.com/1002093
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>