You *might* be able to do this if you installed a paravirtualized kernel in ubuntu and booted it in PV mode instead of FV mode (PV doesn't technically require a full disk). You'd also have to make sure that you passed the ubuntu partition to the VM as a partition instead of as a full disk. For example, your disk line might look something like this:
disk = [ 'phy:/dev/sda3,sda3,w' ]
However, even with these changes you'd probably still run into problems with configuration settings for video, network, fstab, etc., that will make this a less than ideal thing to do.