CLOUDSTACK-4683: revert 2 commits related to kvm cpu model

commit a761d5ae994fc33f60c1c228efe1861ef22dddac
Author: JijunLiu <jijunlx@gmail.com>
Date:   Mon Aug 12 14:48:08 2013 +0800

    add doc for kvm cpu model

commit 706e23623c3227d92ae498529166ca73d2a2bf4e
Author: JijunLiu <jijunlx@gmail.com>
Date:   Sun Jul 14 11:21:39 2013 +0800

    Add cpu model for kvm guest.Now all the kvm guest's cpu model is 'QEMU Virtual CPU version xxx'.
    This will affect the activation of Windows OS and low performance.
    I add three mode for user to indicate the guest cpu model.add libvirt