docs: Document how libvirt handles companion controllers
    
    The information on companion controllers we give in our documentation is
    rather sparse.  For example, it looks like any controller can be used as
    a companion one.  Also, when using ich9-uhci2, for example, we are able
    to set some sensible defaults, but it might get confusing for the user
    as we don't do that for all controller models.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1069590
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>