Engine: Migration target host labeled UNKNOWN

When we don't find a destination host to migrate to we cannot use the
regular fail message since it expected destination host name for the
audit log message.

Change-Id: I7d2819454d4642632ae5cfff7ba83424b9dd8aa4
Bug-Url: https://bugzilla.redhat.com/965617
Signed-off-by: Shahar Havivi <shavivi@redhat.com>