core: print the correct previous destination host

The previous destination host was shown as "UNKNOWN" in the audit log
for retrying to migrate VM. Now the name of the host that was the
destination of the previous attempt is shown.

Change-Id: If0d1f90a5c895a349cc9e9f964fdb9a840c49545
Bug-Url: https://bugzilla.redhat.com/1150642
Signed-off-by: Arik Hadas <ahadas@redhat.com>