The default behavior is to create a socket file at /tmp/guestfs-inspect.sock.
If XDG_RUNTIME_DIR is set, that will be used instead of /tmp.
One could instead specify GUESTFS_INSPECT_ENDPOINT, which requires the full endpoint, as specified by: https://libzmq.readthedocs.io/en/zeromq3-x/zmq_connect.html
You will likely want to use ipc://, but you can probably use tcp:// or (e)pgm:// if that is appropriate.
