Implicitly create a new VT for each consumer

Instead of requiring the graphics driver to create a fixed set of VTs in advance. This fixes the graphical interface when there is a graphics driver but no boot framebuffer. Previously in that case vesad would exit before it creates any VT and thus no consumer could show anything as their VT was missing. In the future creating new VTs on the fly could also allow a display manager to use a separate VT for each user session.