cpu: Add SandyBridge-IBRS CPU model

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>