Introduce Wireshark dissector plugin which adds support to Wireshark
for dissecting libvirt RPC protocol.
Added following files to build Wireshark dissector from libvirt source
tree.
* tools/wireshark/*: Source tree of Wireshark dissector plugin.

Added followings to configure.ac or Makefile.am.
configure.ac
* --with-wireshark-dissector: Enable support for building Wireshark
  dissector.
* --with-ws-plugindir: Specify wireshark plugin directory that dissector
  will installed.
* Added tools/wireshark/{Makefile,src/Makefile} to  AC_CONFIG_FILES.
Makefile.am
* Added tools/wireshark/ to SUBDIR.