virt: sampling: extract disk latency calculation

This is a preparation patch for the merge
of sampleDisk and sampleDiskLatency

Change-Id: I3f78e2a990aefe6095dd4ff54e21ab40006c0713
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29952
Reviewed-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>