libvirt users - Apr 2014 - LXC vCPU controlling problem

Hi,guys! I've been using libvirt-1.1.1 with LXC1.0.0 to do my CS
homework.But I met a problem when I tried to set vcpu for a LXC.The
/sys/fs/cgroup/cpuset/machine/cpuset.cpus always show 0-3 no matter how
many vcpu I defined in the XML setting file.
But I found the command "virsh -c lxc:/// domaininfo lxc_name" got the
right number as it is defined.
What's the problem there? Is it a bug in libvirt-1.1.1?

Thank you!