cpu: Add SandyBridge-IBRS CPU model
    
    This is a variant of SandyBridge with indirect branch prediction
    protection. The only difference between SandyBridge and SandyBridge-IBRS
    is the added "spec-ctrl" feature.
    
    The SandyBridge-IBRS model in QEMU is a bit different since SandyBridge
    got several additional features since we added it in cpu_map.xml:
        arat, vme, xsaveopt
    
    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>