A few things to check:
- Verify that MAAS has the images you want to deploy in terms of Ubuntu releases and architecture.
- Verify that your VM has a big enough disk for an Ubuntu installation (say, give it 10G)
- Verify in MAAS that the node (VM) hardware is correct, i.e., that MAAS discovered the hardware correctly specially regarding the disk.
- I don't remember if virt-io for disks is supported, so if you have that in your VM, try switching to SCSI or SATA (and recommission or even re-enlist so the new hardware is detected).
- Check in the MAAS server logs if there is something fishy (/var/log/maas/*.log) during the boot. Maybe it's not finding the right image and that's the right failure.
If you change the hardware of the VM, don't forget to recommission, or even re-enlist if you suspect it wasn't updated in MAAS.
Other than that, you will need to supply some logs or screenshots.
Hope this helps.