KVM: PPC: Disable KVM on little endian builds

There are a number of KVM issues with little endian builds.
We are working on fixing them, but in the meantime disable
it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>