[libvirt] TSC scaling interface to management

HW TSC scaling is a feature of AMD processors that allows a multiplier to be specified to the TSC frequency exposed to the guest. KVM also contains provision to trap TSC ("KVM: Infrastructure for software and hardware based TSC rate scaling" cc578287e3224d0da) or advance TSC frequency. This is useful when migrating to a host with different frequency and the guest is possibly using direct RDTSC instructions for purposes other than measuring cycles (that is, it previously calculated cycles-per-second, and uses that information which is stale after migration). "qemu-x86: Set tsc_khz in kvm when supported" (e7429073ed1a76518) added support for tsc_khz= option in QEMU. I am proposing the following changes so that management applications can work with this: 1) New option for tsc_khz, which is tsc_khz=host (QEMU command line option). Host means that QEMU is responsible for retrieving the TSC frequency of the host processor and use that. Management application does not have to deal with the burden. 2) New subsection with tsc_khz value. Destination host should consult supported features of running kernel and fail if feature is unsupported. It is not necessary to use this tsc_khz setting with modern guests using paravirtual clocks, or when its known that applications make proper use of the time interface provided by operating systems. On the other hand, legacy applications or setups which require no modification and correct operation while virtualized and make use of RDTSC might need this. Therefore it appears that this "tsc_khz=auto" option can be specified only if the user specifies so (it can be a per-guest flag hidden in the management configuration/manual). Sending this email to gather suggestions (or objections) to this interface.