Client (Win): if 32 bit client on 64 bit host, warn about VirtualBox

The 32-bit client won't be able to detect VBox on a 64-bit host.
Say this in a startup message.

client/client_state.cpp

@@ -242,6 +242,17 @@ void CLIENT_STATE::show_host_info() {
             "VirtualBox version: %s",
             host_info.virtualbox_version
         );
+    } else {
+#if defined (_WIN32) && !defined(_WIN64)
+        if (!strcmp(get_primary_platform(), "windows_x86_64")) {
+            msg_printf(NULL, MSG_INFO,
+                "VirtualBox: can't detect because this is a 32-bit client"
+            );
+            msg_printf(NULL, MSG_INFO,
+                "  (to use VirtualBox, install a 64-bit BOINC client)."
+            );
+        }
+#endif
     }
 }