tools: add simple vchan-socket-proxy

tools: add simple vchan-socket-proxy

Add a simple proxy for tunneling socket connection over vchan. This is
based on existing vchan-node* applications, but extended with socket
support. vchan-socket-proxy serves both as a client and as a server,
depending on parameters. It can be used to transparently communicate
with an application in another domian that normally expose UNIX socket
interface. Specifically, it's written to communicate with qemu running
within stubdom.

Server mode listens for vchan connections and when one is opened,
connects to a pointed UNIX socket.  Client mode listens on UNIX
socket and when someone connects, opens a vchan connection.  Only
a single connection at a time is supported.

Additionally, socket can be provided as a number - in which case it's
interpreted as already open FD (in case of UNIX listening socket -
listen() needs to be already called). Or "-" meaning stdin/stdout - in
which case it is reduced to vchan-node2 functionality.

Example usage:

1. (in dom0) vchan-socket-proxy --mode=client <DOMID>
    /local/domain/<DOMID>/data/vchan/1234 /run/qemu.(DOMID)

2. (in DOMID) vchan-socket-proxy --mode=server 0
   /local/domain/<DOMID>/data/vchan/1234 /run/qemu.(DOMID)

This will listen on /run/qemu.(DOMID) in dom0 and whenever connection is
made, it will connect to DOMID, where server process will connect to
/run/qemu.(DOMID) there. When client disconnects, vchan connection is
terminated and server vchan-socket-proxy process also disconnects from
qemu.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>