Comment # 13 on bug 939432 from Stefan Dirsch

[    17.468] (EE) systemd-logind: failed to get session: PID 936 does not
belong to any known session

Known issue related to systemd-logind. Still investigated.

[    17.658] (EE) Failed to load module "vboxvideo" (module does not exist, 0)

vboxvideo X driver no available on the system. Not sure why. Is
virtualbox-guest-x11 package installed?

[    17.665] (EE) open /dev/dri/card0: No such file or directory

fallback to modeseting fails for some reason.

[    17.683] (EE) AIGLX: reverting to software rendering

That's because of the fallback to fbdev driver.