One of the OST suite (hc-basic-suite-4.3) is failing with error:   libvirtError: COMMAND_FAILED: INVALID_IPV: 'ipv6' is not a valid backend or is unavailable

Full log: 

 * Create network lago-hc-basic-suite-4-3-net-management: ERROR (in 0:00:11)
07:42:57   # Start nets: ERROR (in 0:00:11)
07:42:57 @ Start Prefix: ERROR (in 0:00:11)
07:42:57 Error occured, aborting
07:42:57 Traceback (most recent call last):
07:42:57   File "/usr/lib/python2.7/site-packages/lago/cmd.py", line 969, in main
07:42:57     cli_plugins[args.verb].do_run(args)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/plugins/cli.py", line 184, in do_run
07:42:57     self._do_run(**vars(args))
07:42:57   File "/usr/lib/python2.7/site-packages/lago/utils.py", line 573, in wrapper
07:42:57     return func(*args, **kwargs)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/utils.py", line 584, in wrapper
07:42:57     return func(*args, prefix=prefix, **kwargs)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/cmd.py", line 271, in do_start
07:42:57     prefix.start(vm_names=vm_names)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/sdk_utils.py", line 50, in wrapped
07:42:57     return func(*args, **kwargs)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/prefix.py", line 1323, in start
07:42:57     self.virt_env.start(vm_names=vm_names)
07:42:57   File "/usr/lib/python2.7/site-packages/lago/virt.py", line 341, in start
07:42:57     net.start()
07:42:57   File "/usr/lib/python2.7/site-packages/lago/providers/libvirt/network.py", line 115, in start
07:42:57     net = self.libvirt_con.networkCreateXML(self._libvirt_xml())
07:42:57   File "/usr/lib64/python2.7/site-packages/libvirt.py", line 4216, in networkCreateXML
07:42:57     if ret is None:raise libvirtError('virNetworkCreateXML() failed', conn=self)
07:42:57 libvirtError: COMMAND_FAILED: INVALID_IPV: 'ipv6' is not a valid backend or is unavailable
07:42:57 + on_exit

Any one has idea what could be the issue?

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