KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw_id()

The find_arch_event() returns a "unsigned int" value,
which is used by the pmc_reprogram_counter() to
program a PERF_TYPE_HARDWARE type perf_event.

The returned value is actually the kernel defined generic
perf_hw_id, let's rename it to pmc_perf_hw_id() with simpler
incoming parameters for better self-explanation.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20211130074221.93635-3-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>