virt: stats: move core static stats in a method

this patch reorganizes the code to move all the
stats which will not change after a VM is booted
into a separate method,

Change-Id: I9b5f6cafe5bc1a9b1b27bf974cdeffdebf09d75a
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26550
Reviewed-by: Dan Kenigsberg <danken@redhat.com>