For converting my Debian boxes I do this. It is nothing fancy. Things you would do with
plain physical systems as well.
I have a VM for preparing VM images where I attach newly created disks, rsyncing all data
from old host to the new image.
Then I shut down the preparation VM, remove the new disk from it.
Create a new VM and attach the disk to that VM.
I boot a rescue CD, fix /etc/fstab and networking, Re-Install grub to the disk.
Done.