lgf_command is defined like this 
`def lgf_command(cmd, ignore_status=True, debug=False, timeout=60):`
And dargs contains "is_disk", so I think it will raise an exception
`TypeError: test() got an unexpected keyword argument 'is_disk'`...is that right?
BTW, in my pullreq #885 there is a commit "virt: Add virt_cmd function in utils_libguestfs module", I did
a small change on lgf_command, lgf_command was changed to
`def lgf_command(cmd, **dargs):` do you think my modification acceptable to you ?