https://bugzilla.redhat.com/show_bug.cgi?id=1049391

When all source CPU XMLs contain just a single CPU model (with a
possibly varying set of additional feature elements),
virConnectBaselineCPU will try to use this CPU model in the computed
guest CPU. Thus, when used on just a single CPU (useful with
VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES), the result will not use a
different CPU model.

If the computed CPU uses the source model, set fallback mode to 'forbid'
to make sure the guest CPU will always be as close as possible to the
source CPUs.