qemu: Give hint about -noTSX CPU model
    
    Because of the microcode update to Haswell/Broadwell CPUs, existing
    domains using these CPUs may fail to start even though they used to run
    just fine. To help users solve this issue we try to suggest switching to
    -noTSX variant of the CPU model:
    
        virsh # start cd
        error: Failed to start domain cd
        error: unsupported configuration: guest and host CPU are not
        compatible: Host CPU does not provide required features: rtm, hle;
        try using 'Haswell-noTSX' CPU model
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>