live merge: fix usage of getLastSample

In commit 359c7b60b6125cae0ac47b64dfcffde1dd91c9b7
I forgot to update the sampleVmJobs code to make
use of the new getLastSample() API.

This broke live merge, and this patch fixes that.

Change-Id: Idbdd3ac44e9341888a80154293c22c6882dd420e
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35620
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>