qemu: Give hint about -noTSX CPU model

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>