cpu: Add Nehalem-IBRS CPU model
    
    This is a variant of Nehalem with indirect branch prediction protection.
    The only difference between Nehalem and Nehalem-IBRS is the added
    "spec-ctrl" feature.
    
    Thus the diff matches QEMU, but the new CPU model itself is different.
    The QEMU's versions of both models contain "vme" feature, while this
    feature is missing in libvirt's models. While we can't change the
    existing Nehalem CPU model, we could add "vme" to Nehalem-IBRS to make
    it similar to QEMU, but doing so would fool our CPU detecting code so
    that any Nehalem CPU with "vme" feature would be detected as
    Nehalem-IBRS CPU without spec-ctrl. Not adding "vme" to Nehalem-IBRS is
    safe as QEMU will just provide the feature anyway, which matches what
    happens with Nehalem (and new enough machine types).
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>