can not run vm with a serial port

environment:
server: centOS 6.5, 3.14.19, x86_64
qemu-kvm: QEMU PC emulator version 0.12.1 (qemu-kvm-0.12.1.2), Copyright (c) 2003-2008 Fabrice Bellard
qemu-system-x86_64 :QEMU emulator version 1.2.0 (qemu-kvm-1.2.0), Copyright (c) 2003-2008 Fabrice Bellard
virt-manager: 0.9.0

VM: centOS 6.5, 3.12.30, x86_64

reproduce step:
1. add serial device
2. select device type: unix socket
                 device parameters: path=/dev/ttyS0
                                                       mode=client mode(connect)
3. run the VM

phenomenon:
Error starting domain: internal error process exited while connecting to monitor: qemu-kvm: -chardev socket,id=charserial0,path=/dev/ttyS0,server,nowait: socket bind failed: Address already in use
qemu-kvm: -chardev socket,id=charserial0,path=/dev/ttyS0,server,nowait: chardev: opening backend "socket" failed


Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 44, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 65, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1114, in startup
    self._backend.create()
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 678, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: internal error process exited while connecting to monitor: qemu-kvm: -chardev socket,id=charserial0,path=/dev/ttyS0,server,nowait: socket bind failed: Address already in use
qemu-kvm: -chardev socket,id=charserial0,path=/dev/ttyS0,server,nowait: chardev: opening backend "socket" failed