vshCommandStringParse: Allow retrieving partial result
    
    In the future, this function is going to be called from
    vshReadlineParse() to provide parsed input for completer
    callbacks. The idea is to allow the callbacks to provide more
    specific data. For instance, for the following input:
    
      virsh # domifaddr --domain fedora --interface <TAB><TAB>
    
    the --interface completer callback is going to be called. Now, it
    is more user friendly if the completer offers only those
    interfaces found in 'fedora' domain. But in order to do that it
    needs to be able to retrieve partially parsed result.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>