KVM: x86: Update "last guest TSC" snapshot prior to enabling IRQs/preemption

When refreshing the last observed guest TSC during a guest time update,
write the snapshot before enabling IRQs, i.e. before enabling preemption.
If the task is migrated between updating the local tsc_timestamp, e.g. to
account for catch-up mode, and setting last_guest_tsc, kvm_arch_vcpu_load()
would set the vCPU's TSC offset using the old last_guest_tsc.

In practice, the bug is largely benign as it's not even strictly necessary
for KVM to refresh last_guest_tsc when updating guest time, as KVM's goal
is purely to prevent the guest from observing time jump backwards, i.e.
super duper strictly speaking, KVM only *needs* to update last_guest_tsc in
the VM-Exit path.

In fact, the update kvm_guest_time_update() in wasn't even added to play
nice with kvm_arch_vcpu_load(), it was added by commit 28e4639adf0c ("KVM:
x86: Fix kvmclock bug") to fix code that no longer exists.  As of commit
28e4639adf0c, kvm_guest_time_update() also consumed last_guest_tsc, to try
and prevent guest time from jumping backwards.  That code was eventually
removed by commit f25e656d31ad ("KVM: x86: fix tsc catchup issue with tsc
scaling"), but the last_guest_tsc update hung around.

Keep the update even though it's technically ok to drop the update, e.g. so
that the tsc_catchup updates aren't lost, and so that the guest won't see a
PV clock timestamp that appears to be in the future.

Link: https://patch.msgid.link/20260826213303.914988-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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