Add vsock: transport

VSOCK sockets allow stream connections for virtual machines guests with the host.

Adding support to D-Bus can help providing guest<->host bus services or p2p communications.

The daemon implementation currently needs --disable-selinux, as VSOCK do not seem to be supported yet. As a reference implementation, I thought it would make sense to still implement it. But the proposal for the new protocl & address is probably the most important part to consider atm.

thanks