feat: vsock support for remote console access

[matttbe]

Having a remote console access can be helpful for different reasons, e.g. an easy way to have multiple terminals, not using a serial which can be slow to print a lot of text, etc.

With this new support, a user can simply use vng like before, and add the new --vsock option:

vng --vsock (...)

Then in another terminal, it can connect to the existing VM:

vng --vsock-connect

Advanced users can set a different CID with --vsock-cid, useful when multiple VMs are started in parallel.

It is also possible to change the command that is executed when connected inside the VM. By default, bash -i is used, but it is possible to pass something else, e.g.

vng --vsock byobu

Or zsh, fish, tmux, etc. Or a script, for example to set env vars, change dir and set other dimensions, etc., e.g. starting the VM with:

$ vng --vsock "${PWD}/console.sh"

... and connecting to it from another terminal with:

$ read -r rows columns <<< "$(stty size)"
$ cat <<-EOF > console.sh
    #! /bin/bash
    stty rows ${rows} columns ${columns}
    cd "\${virtme_chdir}"
    HOME=${HOME}
    byobu
  EOF
$ chmod +x console.sh
$ vng --vsock-connect

Note: on my side, 'bash' is a bit weird: I cannot see the command I'm typing. I didn't try to find a solution for a too long time as I'm usually not using bash.

Regarding the kernel config, two new config are now required: VSOCKETS and VIRTIO_VSOCKETS. They seem light enough.

While at it, fixed a typo in the README file with the --network option.

Depends-on: #195

Link: #151

[matttbe]

Note: because I added the options after others from #195, this PR depends on that. But I can change if needed.

[arighi]

I just left a comment about providing a helper script to initialize the console session, but overall it looks great, I love this!

Feel free to merge this as is, we can improve it later in tree.