sdk: delay vboxvideo module loading

On VirtualBox 5.2, vboxvideo crashes when the resolution changes
a few seconds after boot. It does not occur on VirtualBox 5.1.
This patch blacklists the vboxvideo module so it's not loaded
automatically, instead a systemd unit is requested by graphic.target.
This offers sufficient delay for vboxvideo to be loaded after
the resolution change, avoiding the crash.

Apertis: https://phabricator.apertis.org/T4485


Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.co.uk>

Differential Revision: https://phabricator.apertis.org/D7795