qemu: hotplug: Fix adding USB devices to the driver list

We were unconditionally removing the device from the host list, when it
should only be done on error.

This fixes USB collision detection when hotplugging the same device to
two guests.

src/qemu/qemu_hotplug.c

@@ -1515,9 +1515,9 @@ cleanup:
             virSecurityManagerRestoreHostdevLabel(driver->securityManager,
                                                   vm->def, hostdev, NULL) < 0)
             VIR_WARN("Unable to restore host device labelling on hotplug fail");
+        if (added)
+            virUSBDeviceListSteal(driver->activeUsbHostdevs, usb);
     }
-    if (added)
-        virUSBDeviceListSteal(driver->activeUsbHostdevs, usb);
     if (list && usb &&
         !virUSBDeviceListFind(list, usb) &&
         !virUSBDeviceListFind(driver->activeUsbHostdevs, usb))