virt: stats: move the guest stats in a method

this patch moves the guest statistics gathering in
a separate method.

Change-Id: I83adb6ddc28777c50190229e5da728a3bdb3b24e
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26554
Reviewed-by: Dan Kenigsberg <danken@redhat.com>