tools: Introduce SSH proxy

This allows users to SSH into a domain with a VSOCK device:

  ssh user@qemu/machineName

So far, only QEMU domains are supported AND qemu:///system is
looked for the first for 'machineName' followed by
qemu:///session. I took an inspiration from Systemd's ssh proxy
[1] [2].

To just work out of the box, it requires (yet unreleased) systemd
to be running inside the guest to set up a socket activated SSHD
on the VSOCK. Alternatively, users can set up the socket
activation themselves, or just run a socat that'll forward vsock
<-> TCP communication.

1: https://github.com/systemd/systemd/blob/main/src/ssh-generator/ssh-proxy.c
2: https://github.com/systemd/systemd/blob/main/src/ssh-generator/20-systemd-ssh-proxy.conf.in

Resolves: #579


Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>