KVM: PPC: Disable NX for old magic page using guests

Old guests try to use the magic page, but map their trampoline code inside
of an NX region.

Since we can't fix those old kernels, try to detect whether the guest is sane
or not. If not, just disable NX functionality in KVM so that old guests at
least work at all. For newer guests, add a bit that we can set to keep NX
functionality available.

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