KVM: selftests: Increase robustness of LLC cache misses in PMU counters test

Currently the PMU counters test does a single CLFLUSH{,OPT} on the loop's
code, but due to speculative execution this might not cause LLC misses
within the measured section.

Instead of doing a single flush before the loop, do a cache flush on each
iteration of the loop to confuse the prediction and ensure that at least
one cache miss occurs within the measured section.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
[sean: keep MFENCE, massage changelog]
Link: https://lore.kernel.org/r/20240628005558.3835480-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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