vsh: Call vshCmdOptDef completer
    
    Now that we have everything prepared we can call options'
    completer again. At the same time, pass partially parsed input to
    the completer callback - it will help the callbacks to narrow
    down the list of returned options based on user's input. For
    instance, if the completer is supposed to return list of
    interfaces depending on user input it may return just those
    interfaces defined for already specified domain. Of course,
    completers might ignore this parameter.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>