KVM: powerpc: move powerpc-specific structs to uapi/asm/kvm.h

While this in principle breaks the appearance of KVM_PPC_* ioctls on architectures
other than powerpc, this seems unlikely to be a problem considering that there are
already many "struct kvm_ppc_*" definitions in arch/powerpc/include/uapi.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>