remote usb3.0 redir failed, when guest os has more than one vcpus

I was testing usb3.0 redirection, it worked fine with one vcpu.
But when I set vcpu to a number greater than one, the redirection failed.

Host and guest are Fedora 20 and Windows 7.
Client is remote-viwer.

version: qemu 2.3.0  libvirt 1.2.15 spice 0.12.5

libvirt xml: 

<vcpu placement='static'>2</vcpu>
<controller type='usb' index='0' model='nec-xhci'>
</controller>