Live Merge: improve log messages

Make it easier to debug live merge flows by reporting the job ID
whenever it is available.  This will make the messages more istent and
easy to correlate.

Change-Id: Ib5d22e4c503d5781eb1dc00c6c326146d5bbbbd1
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29310
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>