eliminate sampling.BaseSample

For quite some time, BaseSample is not a common base class of anything.
It serves no meaningful abstraction, so this patch inlines into its only
user.

Change-Id: Ia1668e89ac21229cea1d5aac7ea155882f4295a0
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29471
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>