virt: stats: make _calcDiskRate a function

used to be a method, but never uses 'self',
so make it a function.

No changes in behaviour, except the drop of the
unused 'self'.

Change-Id: I7f89f52309afc900a166b761e4041a371d217968
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33736
Reviewed-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>