piix4_smbus 0000.00.07.0: SMBus base address uninitialized - upgrade bios or use force_addr=0xaddr

It doesn’t actually do any harm to the guest (as far as I can tell), so one could perfectly well leave it alone. But it’s possible to get rid of it altogether simply by issuing three commands and editing one configuration file. First:

sudo nano /etc/modprobe.d/blacklist.conf

Add the text

blacklist i2c_piix4

…to the end of the file. Finally, issue the commands:

sudo update-initramfs -u -k all
sudo reboot

When the machine comes back up after that last reboot, you won’t see the error message. What this is doing is simply preventing an attempt to load the i2c_piix4 kernel module. VirtualBox doesn’t emulate the hardware for which this module is actually needed, but Ubuntu doesn’t know that and tries to load it anyway. Blacklisting the module prevents Ubuntu doing that and thus makes the error disappear.