I'm trying to properly install virtualbox guest additions to a guest linux Debian virtual machine.
The problem is that video performance, at 1920x1080 is crap.
The host is a macOS core i5 with a radeon R9 M395.

I've installed all virtualbox-guest packages:
`virtualbox-guest-x11 virtualbox-guest-dkms virtualbox-guest-addittions`    

After looking inside /var/log/Xorg.0.log it seems that vboxvideo module is not loading. 

    $ cat /var/log/Xorg.0.log
    ...
    [    15.387] (II) LoadModule: "vboxvideo"
    [    15.388] (WW) Warning, couldn't open module vboxvideo
    [    15.388] (II) UnloadModule: "vboxvideo"
    [    15.388] (II) Unloading vboxvideo
    [    15.388] (EE) Failed to load module "vboxvideo" (module does not exist, 0)
    ...

    $ dkms stat
    virtualbox-guest, 5.1.12, 4.8.0-2-amd64, x86_64: installed
    $ uname -r
    4.8.0-2-amd64
    $ lsmod| grep vbox
    vboxsf                 49152  0
    vboxvideo              49152  2
    vboxguest             282624  6 vboxsf,vboxvideo
    ttm                    98304  1 vboxvideo
    drm_kms_helper        155648  1 vboxvideo
    drm                   360448  5 vboxvideo,ttm,drm_kms_helper

What's happening?

Where could I locate vboxvideo module? What is the filename?

Where shall I look next?