KVM: x86: Upscale TSC to "now", not master clock when updating PV clocks

When doing TSC upscaling in software, e.g. when the guest TSC frequency is
configured to run faster than the host without hardware support, always
advance guest TSC to "now", not to the master clock's reference values,
which represents some time in the past.

In master clock mode, there are two points of time which need to be taken
into account. First there is the master clock reference point, stored in
kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now).
Secondly, there is the time *now*, at the point kvm_update_guest_time()
is being called.

With software TSC upscaling, the guest TSC is getting further and further
ahead of the host TSC as time elapses.  So at time "now", the guest TSC
should be further ahead of the host, than it was at master_kernel_ns.
Not taking that into account means KVM was advancing the guest TSC only by
the amount appropriate for the snapshot taken in the past, *not* to the
current time (which is what RDTSC is supposed to return).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20260826213303.914988-17-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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