Recently we had a CentOS virtual machine on VirtualBox which we wanted to use in GNOME Boxes.
We copied the .vdi disk image and we used it to create a new virtual machine in Boxes.
Note A:
By doing this we realized that the system did not reuse the .vdi image.
It merely created a copy at ~/.local/share/gnome-boxes/images/ that was suitable for GNOME Boxes.
So, be sure to have enough space when doing an import like this.
You will need at least twice the space of the .vdi image to complete the migration.
Note B:
When the guest OS started the window manager crashed and it did not allow us to login.
We assumed that this issue occurred due to the VirtualBox Guest Additions that were installed on the guest OS.
As we could not login with the graphical interface, we could not verify this claim.
Fortunately, CentOS (and many other Linux distributions) allow you to switch to a console login using the key combination alt + ctrl + F3.
(There are more than one valid key combinations to do this. In some systems alt + ctrl + F4 is also valid or alt + ctrl + F1 etc).
We hoped that by trying to login via a console login, the Guest Additions would not start and the system would not crash, which luckily this was the case, and we managed to login through the console!
After we logged in, we had to remove the Guest additions. To do so we had to execute the uninstall script that was located at /opt/VBoxGuestAdditions-X.Y.Z/uninstall.sh
(X.Y.Z is the version number of the installed VirtualBox Guest Additions).
When the removal was complete, we executed sudo reboot to restart the system and unload any VirtualBox services that could be executing at the time.
Once the system completed the restart we were able to login properly from the GUI of GNOME and use our virtual machine properly.