usb: make attach optional.

Add a auto_attach field to USBDevice, which is enabled by default.
USB drivers can clear this field in case they do *not* want the device
being attached (i.e. plugged into a usb port) automatically after
successfull init().

Use cases (see next patches):
 * attaching encrypted mass storage devices.
 * -usbdevice host:...
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/usb-bus.c

@@ -45,8 +45,9 @@ static int usb_qdev_init(DeviceState *qdev, DeviceInfo *base)
 
     pstrcpy(dev->devname, sizeof(dev->devname), qdev->info->name);
     dev->info = info;
+    dev->auto_attach = 1;
     rc = dev->info->init(dev);
-    if (rc == 0)
+    if (rc == 0 && dev->auto_attach)
         usb_device_attach(dev);
     return rc;
 }

hw/usb.h

@@ -133,6 +133,7 @@ struct USBDevice {
     int speed;
     uint8_t addr;
     char devname[32];
+    int auto_attach;
     int attached;
 
     int state;