usb-hub can not be detached when detach usbdevice VM

[Michael liu]

I give a host usb device to guest in the way of passthrough,use "virsh attach-device" cmd. In guest os,use "lsusb" cmd I can see two devices have been added,one is usb device and the other is usb-hub（0409:55aa NEC Corp. Hub）.
when I use "virsh detach-device" detach the usb device,in guest os the usb-hub was still exists.
It can create a bad impression when operating the VM,for example,suspend and resume the VM,qemu would report that:

2016-05-24T12:03:54.434369Z qemu-kvm: Unknown savevm section or instance '0000:00:01.2/2/usb-hub' 0

2016-05-24T12:03:54.434742Z qemu-kvm: load of migration failed: Invalid argument

From qemu's code,it can be sure that the usb-hub is generated by qemu,and the process of detaching usb-hub has already been executed,but failed.With adding print information,error as follows:
libusbx: error [do_close] Device handle closed while transfer was still being processed, but the device is still connected as far as we know
libusbx: warning [do_close] A cancellation for an in-flight transfer hasn't completed but closing the device handle