zb: Add support for AF_VSOCK
The "vsock" transport isn't yet accepted by the D-Bus maintainers and none of the existing brokers support this (yet) but it's already usable in two cases:
- p2p
- Firecracker's vsock support, where the host side is a normal unix socket and hence a D-Bus broker can be bound on that and a zbus-based app or service in the guest can connect through this new vsock support.
This work is based on the WIP MR (!229 (closed)) by @elmarco.