KVM: arm64: Use symbolic names for the PMU versions

Instead of using a bunch of magic numbers, use the existing definitions
that have been added since 8673e02e ("arm64: perf: Add support
for ARMv8.5-PMU 64-bit counters")

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>