docs: Add mist.io as libvirt-based application
    
    As reported on the libvirt-users list [1], there's new web
    application called mist.io which uses libvirt as one of its
    backends. Lets add it into our list of libivrt based
    applications.
    
    1: https://www.redhat.com/archives/libvirt-users/2015-February/msg00096.html
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>