KVM: PPC: Book3S HV: Access XICS in BE

On the exit path from the guest we check what type of interrupt we received
if we received one. This means we're doing hardware access to the XICS interrupt
controller.

However, when running on a little endian system, this access is byte reversed.

So let's make sure to swizzle the bytes back again and virtually make XICS
accesses big endian.

Signed-off-by: Alexander Graf <agraf@suse.de>