KVM: arm/arm64: Fix MMIO emulation data handling

When the kernel was handling a guest MMIO read access internally, we
need to copy the emulation result into the run->mmio structure in order
for the kvm_handle_mmio_return() function to pick it up and inject the
	result back into the guest.

Currently the only user of kvm_io_bus for ARM is the VGIC, which did
this copying itself, so this was not causing issues so far.

But with the upcoming new vgic implementation we need this done
properly.

Update the kvm_handle_mmio_return description and cleanup the code to
only perform a single copying when needed.

Code and commit message inspired by Andre Przywara.

Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>