Steps to reproduce this bug:
# cat usb.xml
<hostdev mode='subsystem' type='usb'>
<source>
<address bus='0x001' device='0x003'/>
</source>
</hostdev>
# virsh attach-device vm1 usb.xml
error: Failed to attach device from usb.xml
error: server closed connection:
The reason of this bug is that we set data.cgroup to NULL, and this will cause
libvirtd crashed.
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>