RISC-V: KVM: Fix perf-backed counter accounting across stop and read

pmu_ctr_read() adds the event count returned by perf_event_read_value()
to counter_val, which can accumulate the same count repeatedly across
reads. kvm_riscv_vcpu_pmu_ctr_stop() also leaves counter_val stale by
not folding the current event count into it.

Make reads of perf-backed counters side-effect free, and use
perf_event_pause() when stopping a counter to fold the current event
count into counter_val while resetting it. This preserves the counter
value across stop/start and lets the snapshot path use counter_val
directly.

Fixes: 0cb74b65d2e5 ("RISC-V: KVM: Implement perf support without sampling")
Signed-off-by: SeungJu Cheon <suunj1331@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260825083719.643970-4-suunj1331@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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