virt: sampling: use getLastSample

This patch makes use of the new
getLastSample method when a sample
window is not needed.

No intentional changes in behaviour.

Change-Id: Icef44436256d100836b1d262bb348bb39ee5e647
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33780
Reviewed-by: Dan Kenigsberg <danken@redhat.com>