x86: add platform hypercall to retrieve pCPU-s' family, model, and stepping

x86: add platform hypercall to retrieve pCPU-s' family, model, and stepping

With the recent hotplug changes to the Xen part of the microcode
loading, this allows the kernel driver to avoid unnecessary calls into
the hypervisor during pCPU hot-enabling: Knowing that the hypervisor
retains the data for already booted CPUs, only data for CPUs with a
different signature needs to be passed down. Since the microcode
loading code can be pretty verbose, avoiding to invoke it can make the
log much easier to look at in case of problems.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>