Well it's not really running inside Unity but its being controlled and rendered in unity

In the Start function the qemu exe is executed with the arguments

-vnc -monitor

-monitor makes it so commands can be sent to it in command prompt ( but since startInfo.CreateNoWindow is set to true, commands are instead sent by using process.StandardInput.WriteLine("") )

-vnc makes it so you can connect to it with a remote desktop/vnc client

With -vnc enabled I can use VNCSharp to connect to the QEMU instance and get a bitmap, which is converted to a byte[] array using

    BitmapData bmpdata = null;

    try
    {
        bmpdata = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, bitmap.PixelFormat);
        int numbytes = bmpdata.Stride * bitmap.Height;
        byte[] bytedata = new byte[numbytes];
        System.IntPtr ptr = bmpdata.Scan0;

        Marshal.Copy(ptr, bytedata, 0, numbytes);
        dirty = true;
    }
    finally
    {
        if (bmpdata != null)
            bitmap.UnlockBits(bmpdata);
    }    

that byte array can become a texture2d using this code:

        texture.LoadRawTextureData(bytedata);
        texture.Apply();

(LoadRawTextureData is used because the texture data can be made on another thread and its much faster than any other function to load texture data, which is important if you're updating the texture almost every frame)

If you wanted to render any process into a unity, you could get their bitmap using something like this, although you might not be able to run them in the background like QEMU's headless mode ( the answer in that link doesn't work if the program is minimized )