usb: Add QOM property "attached".

USB devices in attached state are visible to the guest.  This patch adds
a QOM property for this.  Write access is opt-in per device.  Some
devices manage attached state automatically (usb-host, usb-serial,
usb-redir), so we can't enable write access universally but have to do
it on a case by case base.  So far, no device opts in.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-id: 1465984019-28963-4-git-send-email-kraxel@redhat.com

[ minor codestyle fix ]
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/bus.c

@@ -736,6 +736,48 @@ USBDevice *usbdevice_create(const char *cmdline)
     return dev;
 }
 
+static bool usb_get_attached(Object *obj, Error **errp)
+{
+    USBDevice *dev = USB_DEVICE(obj);
+
+    return dev->attached;
+}
+
+static void usb_set_attached(Object *obj, bool value, Error **errp)
+{
+    USBDevice *dev = USB_DEVICE(obj);
+    Error *err = NULL;
+
+    if (dev->attached == value) {
+        return;
+    }
+
+    if (value) {
+        usb_device_attach(dev, &err);
+        if (err) {
+            error_propagate(errp, err);
+        }
+    } else {
+        usb_device_detach(dev);
+    }
+}
+
+static void usb_device_instance_init(Object *obj)
+{
+    USBDevice *dev = USB_DEVICE(obj);
+    USBDeviceClass *klass = USB_DEVICE_GET_CLASS(dev);
+
+    if (klass->attached_settable) {
+        object_property_add_bool(obj, "attached",
+                                 usb_get_attached, usb_set_attached,
+                                 NULL);
+    } else {
+        object_property_add_bool(obj, "attached",
+                                 usb_get_attached, NULL,
+                                 NULL);
+    }
+}
+
 static void usb_device_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *k = DEVICE_CLASS(klass);
@@ -749,6 +791,7 @@ static const TypeInfo usb_device_type_info = {
     .name = TYPE_USB_DEVICE,
     .parent = TYPE_DEVICE,
     .instance_size = sizeof(USBDevice),
+    .instance_init = usb_device_instance_init,
     .abstract = true,
     .class_size = sizeof(USBDeviceClass),
     .class_init = usb_device_class_init,

include/hw/usb.h

@@ -347,6 +347,7 @@ typedef struct USBDeviceClass {
 
     const char *product_desc;
     const USBDesc *usb_desc;
+    bool attached_settable;
 } USBDeviceClass;
 
 typedef struct USBPortOps {