virt-viewer: Support newer libvirt xml format
    
    Since libvirt 0.9.4 there is a new listen element which can be used
    to specify address instead of using the attributes of graphics element.
    
    Also add support for listen type socket - available for Qemu since
    libvirt 2.0.0
    
    Resolves: rhbz#1411765
    
    Acked-by: Jonathon Jongsma <jjongsma@redhat.com>