KVM: x86/pmu: Calculate set of to-be-emulated PMCs at time of WRMSRs

Calculate and track PMCs that are counting instructions/branches retired
when the PMC's event selector (or fixed counter control) is modified
instead evaluating the event selector on-demand.  Immediately recalc a
PMC's configuration on writes to avoid false negatives/positives when
KVM skips an emulated WRMSR, which is guaranteed to occur before the
main run loop processes KVM_REQ_PMU.

Out of an abundance of caution, and because it's relatively cheap, recalc
reprogrammed PMCs in kvm_pmu_handle_event() as well.  Recalculating in
response to KVM_REQ_PMU _should_ be unnecessary, but for now be paranoid
to avoid introducing easily-avoidable bugs in edge cases.  The code can be
removed in the future if necessary, e.g. in the unlikely event that the
overhead of recalculating to-be-emulated PMCs is noticeable.

Note!  Deliberately don't check the PMU event filters, as doing so could
result in KVM consuming stale information.

Tracking which PMCs are counting branches/instructions will allow grabbing
SRCU in the fastpath VM-Exit handlers if and only if a PMC event might be
triggered (to consult the event filters), and will also allow the upcoming
mediated PMU to do the right thing with respect to counting instructions
(the mediated PMU won't be able to update PMCs in the VM-Exit fastpath).

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20250805190526.1453366-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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