`vhost-device-vsock`[1] is a custom implementation of AF_VSOCK, but the
application on the host-side uses a UNIX domain-socket. This gives us
the following nice properties:

* We don't need to do `--arg sandbox-paths /dev/vhost-vsock` anymore for
  debugging builds within the sandbox. That means, untrusted users can
  also debug these kinds of tests now.

* This prevents CID conflicts on the host-side, i.e. there's no need for
  using `sshBackdoor.vsockOffset` for tests anymore.

A big shout-out goes to Allison Karlitskaya, the developer of test.thing[2]
who talked about this approach to do AF_VSOCK on All Systems Go 2025.

This patch requires systemd 258[3] because this contains `vhost-mux` in
its SSH config which is needed to connect to the VMs from now on.

To not blow up the patches even more, this only uses AF_VSOCK for the
debugger. A potential follow-up for the future would be a removal of the
current `backdoor.service` and replace it entirely by this
functionality.

The internal implementation tries to be consistent with how VLANs and
machines are handled, i.e. the processes are started when the Driver's
context is entered and cleaned up in __exit__().

I decided to push the process management and creation of sockets for
vhost-device-vsock into its own class, that's an implementation detail
and not a concern for the test-driver. In fact, `vhost-device-vsock` is
something we can drop once QEMU implements native support for using
AF_UNIX on the host-side[4]. `VsockPair` is its own class since
returning e.g. a triple of `(Path, Path, Int)` would be ambiguous in
what is the guest and what the host path (and frankly, I found it hard
to distinguish the two when reading the docs of `vhost-device-vsock`
initially).

Finally, now that we can do the SSH backdoor without adding additional
devices to the sandbox, I figured, it's time to write a test-case for
it.

[1] https://github.com/rust-vmm/vhost-device/blob/main/vhost-device-vsock/README.md
[2] https://codeberg.org/lis/test.thing
[3] NixOS#427968
[4] https://gitlab.com/qemu-project/qemu/-/issues/2095