I try to configure libvirtd to use qemu+tcp, but after doing so, I can't use virsh or programs with libvirt API. 

I type "virsh -c qemu:///system", then 
error: unable to connect to '/var/run/libvirt/libvirt-sock', libvirtd may need to be started: Permission denied
error: failed to connect to the hypervisor

I could add "sudo" to "virsh -c qemu:///system", running virsh with "qemu:///system" normally, but it's not suitable for my requests. 

Also I try to restart or reinstall libvirt-bin in Ubuntu, also failed.

Could you give me some idea?

Thanks a lot!