KVM: PPC: Book3S: Stop PTE lookup on write errors

When a page lookup failed because we're not allowed to write to the page, we
should not overwrite that value with another lookup on the second PTEG which
will return "page not found". Instead, we should just tell the caller that we
had a permission problem.

This fixes Mac OS X guests looping endlessly in page lookup code for me.

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