conn = libvirt.open("vbox:///session")

return this error:

libvir: VBOX error : internal error unable to initialize VirtualBox driver API
Traceback (most recent call last):
  File "prova.py", line 5, in <module>
    conn = libvirt.open("vbox:///session")
  File "/usr/lib/python2.6/dist-packages/libvirt.py", line 163, in open
    if ret is None:raise libvirtError('virConnectOpen() failed')
libvirt.libvirtError: internal error unable to initialize VirtualBox driver API

why?