allow if=none for drive_add

Allow adding unconnected host drives by specifying if=none like it is
possible with -drive.  They can be put in use with drive attributes,
like this:

  drive_add dummy if=none,id=mydisk,file=/some/disk.img
  device_add virtio-blk-pci,drive=mydisk

which is the monitor aequivalent to these command line switches:

  -drive if=none,id=mydisk,file=/some/disk.img
  -device virtio-blk-pci,drive=mydisk
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/pci-hotplug.c

@@ -91,6 +91,9 @@ void drive_hot_add(Monitor *mon, const QDict *qdict)
                        dinfo->bus,
                        dinfo->unit);
         break;
+    case IF_NONE:
+        monitor_printf(mon, "OK\n");
+        break;
     default:
         monitor_printf(mon, "Can't hot-add drive to type %d\n", type);
         goto err;