KVM: arm64: Factor out fault info population and gic workarounds

The current world-switch function has functionality to detect a number
of cases where we need to fixup some part of the exit condition and
possibly run the guest again, before having restored the host state.

This includes populating missing fault info, emulating GICv2 CPU
interface accesses when mapped at unaligned addresses, and emulating
the GICv3 CPU interface on systems that need it.

As we are about to have an alternative switch function for VHE systems,
but VHE systems still need the same early fixup logic, factor out this
logic into a separate function that can be shared by both switch
functions.

No functional change.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>