make cr3_guard non-copyable, but movable

src/hvpp/hvpp/lib/cr3_guard.h

@@ -18,6 +18,9 @@
 // since we don't have control over Windows' internal PFN lock, the return
 // value of this function might be unreliable.
 //
+// Note:
+//   This class is non-copyable, but movable.
+//
 
 namespace detail
 {
@@ -31,8 +34,20 @@ class cr3_guard
       : previous_cr3_{ ia32::read<ia32::cr3_t>() }
     { ia32::write<ia32::cr3_t>(::detail::kernel_cr3(new_cr3)); }
 
+    cr3_guard(const cr3_guard& other) noexcept = delete;
+
+    cr3_guard(cr3_guard&& other) noexcept
+      : previous_cr3_{ other.previous_cr3_ }
+    { other.previous_cr3_.flags = 0; }
+
     ~cr3_guard() noexcept
-    { ia32::write<ia32::cr3_t>(previous_cr3_); }
+    { if (previous_cr3_.flags) ia32::write<ia32::cr3_t>(previous_cr3_); }
+
+    cr3_guard& operator=(const cr3_guard& other) noexcept = delete;
+
+    cr3_guard& operator=(cr3_guard&& other) noexcept
+    { previous_cr3_ = other.previous_cr3_;
+      other.previous_cr3_.flags = 0; }
 
   private:
     ia32::cr3_t previous_cr3_;