"network ready" notification

[cfergeau]

When gvproxy is started, it's hard to know when everything is initialized and when it's ready to process networking.
Having a way to get a notification (on a socket?) when gvproxy is ready would help with this. Maybe we can use the same channel to notify when a VM connected to it, when an address was assigned through DHCP, ..

[cfergeau]

I’d send notifications:

• when gvproxy is waiting for connections from the hypervisor
• when a connection with the hypervisor was successfully created
• when there was an error trying to make that connection
• when that connection is closed (if possible)
• when the VM gets an address through dhcp

The transport for this is to be defined, but json messages over a unix socket should be good enough for now if we don’t have a better idea.