core: clear migrating_to_vds after successful migration

We used to clear this field only upon failed migration, now it will be
clear after successful migration as well.

Change-Id: Icb4c73eba707551259cdfc3ca6cacb7b61ff5594
Bug-Url: https://bugzilla.redhat.com/1112359
Signed-off-by: Arik Hadas <ahadas@redhat.com>