KVM: arm64: Upgrade PMU support to ARMv8.4

Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
pretty easy. All that is required is support for PMMIR_EL1, which
is read-only, and for which returning 0 is a valid option as long
as we don't advertise STALL_SLOT as an implemented event.

Let's just do that and adjust what we return to the guest.

Signed-off-by: Marc Zyngier <maz@kernel.org>