KVM: x86/pmu: Get eventsel for fixed counters from perf

KVM: x86/pmu: Get eventsel for fixed counters from perf

Get the event selectors used to effectively request fixed counters for
perf events from perf itself instead of hardcoding them in KVM and hoping
that they match the underlying hardware.  While fixed counters 0 and 1 use
architectural events, as of ffbe4ab0beda ("perf/x86/intel: Extend the
ref-cycles event to GP counters") fixed counter 2 (reference TSC cycles)
may use a software-defined pseudo-encoding or a real hardware-defined
encoding.

Reported-by: Kan Liang <kan.liang@linux.intel.com>
Closes: https://lkml.kernel.org/r/4281eee7-6423-4ec8-bb18-c6aeee1faf2c%40linux.intel.com
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>