KVM: arm64: Rename kvm_pmu_valid_counter_mask()

Nested PMU support requires dynamically changing the visible range of
PMU counters based on the exception level and value of MDCR_EL2.HPMN. At
the same time, the PMU emulation code needs to know the absolute number
of implemented counters, regardless of context.

Rename the existing helper to make it obvious that it returns the number
of implemented counters and not anything else.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241025182354.3364124-13-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>