virt: sampling: extract disk rate calculation

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

Change-Id: I9623b1b4ecbbe4726b7642e91e3c45ef743d9dfe
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29951
Reviewed-by: Dan Kenigsberg <danken@redhat.com>