PPC: KVM: Make NX bit available with magic page

Because old kernels enable the magic page and then choke on NXed trampoline
code we have to disable NX by default in KVM when we use the magic page.

However, since commit b18db0b8

 we have successfully fixed that and can now
leave NX enabled, so tell the hypervisor about this.

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