stats: change log to exception for missing stats

Once we have explicitely handled the common error
conditions for missing stats, we should now consider
really 'exceptional' the case of a missing stat.

Change-Id: Ic2655dc84c063a2dde43742647b02dba4066312b
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27419
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>