The SMBus is for information such as motherboard temperature and there isn't any on a virtual machine (as far as I am aware), but Linux still starts the driver for it. I am fairly sure it isn't related to your GUI issue.
Sorry if this sounds Mickey Mouse, but did you install Ubuntu workstation or server? The server hasn't got a GUI by default. Workstations put the GUI in terminal 7. You can switch terminal screens by pressing CTRL+ALT+F7 (F1 to F7 selects the different screens). So when your VM starts, try pressing CTRL+ALT+F7 and see what's on the screen. If it's just another logon prompt, then the GUI isn't working or not installed. Log in and try typing startx to see if the GUI strats. If not, check the messages you get and let us know.
If you've installed server and want a GUI, there are quite a few to be had. Some are lightweight and some are extremely heavy. One of the lighter is xfce4.