virt: stats: move vm runtime stats in a method

this patch gathers all the stats values which
can change during the life o a VM in a separate
method.

Change-Id: I5a2b7658c209a3dcdd6df46052440204ad52a5d3
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26548
Reviewed-by: Dan Kenigsberg <danken@redhat.com>