docs: network diagrams for the wiki

docs: network diagrams for the wiki

We provide two new diagrams
  docs/figs/network-{bridge,basic}.fig
which are converted to pngs by the Makefiles and intended for
consumption by http://wiki.xen.org/wiki/Xen_Networking.

This is perhaps not the ideal location for this source code but we
don't have a better one.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>