[libvirt-users] Unable to connect qemu:///system on CentOS 6.2 X86_64

Dear all: I ust installed Fedora16Fedora16 X86_64 on my new machine, and then I ran the commands as follows(ran as root): yum install qemu-kvm yum install qemu yum install libvirt yum install virt-manager then I started the libvirtd as follow: /etc/init.d/lbvirtd start So , it ran well. However, after a few days , when I ran the command "virt-manager" , it showed as in the attachment. it seemed that ti cannot connect the hipervisor. Then I did a little test: I ran the conmand "virsh list" , but it didnot work,either. Then I used Python as follows: import libvirt cc = libvirt.open(r'qemu:///system' ) but it didnot work,either. All the test above are under root . I hope that someone can give me some suggestiion. Thanks to all!