KVM: arm64: Refactor PMU attribute error handling

The PMU emulation error handling is pretty messy when dealing with
attributes. Let's refactor it so that we have less duplication,
and that it is easy to extend later on.

A functional change is that kvm_arm_pmu_v3_init() used to return
-ENXIO when the PMU feature wasn't set. The error is now reported
as -ENODEV, matching the documentation. -ENXIO is still returned
when the interrupt isn't properly configured.

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