cpu: Add Westmere-IBRS CPU model
    
    This is a variant of Westmere with indirect branch prediction
    protection. The only difference between Westmere and Westmere-IBRS is
    the added "spec-ctrl" feature.
    
    The Westmere-IBRS model in QEMU is a bit different since Westmere got
    several additional features since we added it in cpu_map.xml:
        arat, pclmuldq, vme
    
    Adding them only to the -IBRS variant would confuse our CPU detection
    code.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>