KVM: x86: Update Xen TSC leaves during CPUID emulation

The Xen emulation in KVM modifies certain CPUID leaves to expose
TSC information to the guest.

Previously, these CPUID leaves were updated whenever guest time changed,
but this conflicts with KVM_SET_CPUID/KVM_SET_CPUID2 ioctls which reject
changes to CPUID entries on running vCPUs.

Fix this by updating the TSC information directly in the CPUID emulation
handler instead of modifying the vCPU's CPUID entries.

Signed-off-by: Fred Griffoul <fgriffo@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20250124150539.69975-1-fgriffo@amazon.co.uk


Signed-off-by: Sean Christopherson <seanjc@google.com>