VDAgentConnection
Sent back in 30-April-2019 but not reviewed yet.
finally sending v2 of the series that Victor reviewed back in December.
Main changes since v1:
- tried to improve the design --> udscs and virtio_port are now implemented as subclasses of VDAgentConnection
- reimplemented write in VDAgentConnection so that flush is now synchronous (doesn't use the "hack" with GMainLoop)