KVM: guest_memfd: Optimize away conversion overheads via dead-code elimination

引入kvm_arch_has_gmem_convert()宏来守卫guest_memfd调用arch转换钩子，x86只有一半需要运行时钩子，通过死代码消除去掉空调开销，并提供kvm_arch_gmem_make_private声明。