KVM: arm64: Only read HPFAR_EL2 when value is architecturally valid

KVM's logic for deciding when HPFAR_EL2 is UNKNOWN doesn't align with
the architecture. Most notably, KVM assumes HPFAR_EL2 contains the
faulting IPA even in the case of an SEA.

Align the logic with the architecture rather than attempting to
paraphrase it. Additionally, take the opportunity to improve the
language around ARM erratum #834220 such that it actually describes the
bug.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250402201725.2963645-2-oliver.upton@linux.dev


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