wireshark: Install to generic plugin directory
    
    There has been a report on the list [1] that we are not
    installing the wireshark dissector into the correct plugin
    directory. And in fact we are not. The problem is, the plugin
    directory path is constructed at compile time. However, it's
    dependent on the wireshark version, e.g.
    
      /usr/lib/wireshark/plugins/1.12.6
    
    This is rather unfortunate, because if libvirt RPMs were built
    with one version, but installed on a system with newer one, the
    plugins are not really loaded. This problem lead fedora packagers
    to unify plugin path to:
    
      /usr/lib/wireshark/plugins/
    
    Cool! But this was enabled just in wireshark-1.12.6-4. Therefore,
    we must require at least that version.
    
    And while at it, on some distributions, the wireshark.pc file
    already has a variable that defines where plugin dir is. Use that
    if possible.
    
    1: https://www.redhat.com/archives/libvirt-users/2015-October/msg00063.html
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>