Configure TSC frequency

libvirt currently supports exposing the invTSC falg, in turn it's possible to expose that feature to VMs with 'cpu_model_extra_flags'.
The only issue is that libvirt will only live-migrate VMs with an explicit TSC frequency and that configuring this is currently not possible.

# Not live-migratable
<cpu mode='...' >
    <feature policy='require' name='invtsc'/>
    ...
</cpu>
<clock offset='localtime'>
    <!-- Any clock here, but NO tsc freq -->
</clock>

# Live-migratable
<cpu mode='...' >
    <feature policy='require' name='invtsc'/>
    ...
</cpu>
<clock offset='localtime'>
    <timer name='tsc' frequency='...'/>
    ....
</clock>

Usecase: (Windows guest specific)
 Many games or game engines will look for the invTSC flag and make sure it's present or refuse to start or will not check for it, but behave unreliably if it isn't present. As a cloud operator I can enable the cpu flag, but I would also want to be able to live-migrate such VMs