sampling: rename variables to make code nicer

Rename internal variables of SampleWindow
methods to make the code nicer and easier
to read.

No changes in behaviour.

Change-Id: I14caeb163552a659d1e7fcc103ac7fc9f1697bad
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34390
Reviewed-by: Dan Kenigsberg <danken@redhat.com>