network: add connections counter to networks

network: add connections counter to networks

Just as each physical device used by a network has a connections
counter, now each network has a connections counter which is
incremented once for each guest interface that connects using this
network.

The count is output in the live network XML, like this:

   <network connections='20'>
   ...
   </network>

It is read-only, and for informational purposes only - it isn't used
internally anywhere by libvirt.