vsh: Introduce complete command
    
    This command is going to be called from bash completion script in
    the following form:
    
      virsh complete -- start --domain
    
    Its only purpose is to return list of possible strings for
    completion. Note that this is a 'hidden', unlisted command and
    therefore there's no documentation to it.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>