KVM: guest_memfd: Always fault from guest_memfd if in-place conversion is enabled

in-place转换启用时强制设置KVM_MEMSLOT_GMEM_ONLY，使guest_memfd成为shared和private内存唯一backing源，简化stage2失效；userspace_addr仍用于kvm_read_guest等但需用户保证一致。