Introduce DEVICE_CATEGORY_CPU for CPU devices

Now that CPUs show up in the help text of "-device ?",
we should group them into an appropriate category.

Backports commit ba31cc7226ebcee639f18faa90c1542bd364fba3 from qemu

qemu/include/hw/qdev-core.h

@@ -25,6 +25,7 @@ typedef enum DeviceCategory {
     DEVICE_CATEGORY_DISPLAY,
     DEVICE_CATEGORY_SOUND,
     DEVICE_CATEGORY_MISC,
+    DEVICE_CATEGORY_CPU,
     DEVICE_CATEGORY_MAX
 } DeviceCategory;