wireshark: Install to generic plugin directory

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>