MAINTAINERS: Add an entry for the vboxvideo driver

Add a MAINTAINERS entry for the vboxvideo driver, now that it has been
moved out of staging.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190311163522.22045-1-hdegoede@redhat.com