I understand that you have a single disk image that contains, say the root filesystem of a run-able Ubuntu instance and you want to create more than one virtual machine that will run this Ubuntu disk image.
One option would be to clone your VirtualBox instance.
But, if you want to do this by using the same disk image for multiple virtual machines then you can change the Type of this disk image to Multi-attach.
According to VirtualBox documentation (section 5.4) the disk format (VMDK, VDI, etc.) is irrelevant to this.
Multi-attach mode disk images can be attached to more than one virtual machines at the same time, even if these machines are running simultaneously. For each virtual machine to which such an image is attached, a "differencing" image is created (under the Snapshots directory of the VM's VirtualBox definition directory). As a result, any data that is written to such a virtual disk by one machine is not seen by the other machines to which the image is also attached. Each machine creates its own write history of the multi-attach image and the original disk image is never updated.
A multi-attach type disk is like an immutable disk image, except that the "differencing" image is not reset every time the machine starts. In other words, the changes made by each virtual machine are persistent between VM restarts and this means that the "differencing" image of a VM is going to grow as the VM makes updates on the "multi-attach"ed disk.
Note that according to the documentation,
This mode is useful for sharing files which are almost never written,
for instance picture galleries, where every guest changes only a small
amount of data and the majority of the disk content remains unchanged.
The modified blocks are stored in differencing images which remain
relatively small and the shared content is stored only once at the
host.
IMHO a multi-attach type disk can be used for an operating system's root partition (I haven't tested though). However, I don't see any real advantage over creating clones.
