Luckily, another Clonezilla user already submitted a patch for VirtIO drives back in December. It should make it into a future stable release in a few months.
I was in rush to get a P2V complete, so I used a quick sed onliner to modify stable Clonezilla's Perl scripts to recognize the /dev/vda disk. You'll need to drop into the shell mode to execute this.
sudo sed -i '/\[hs\]/\[vhs\]/' /opt/drbl/sbin/ocs-*
Keep in mind that these changes will be lost if your booting from a Live CDROM.
Using the VirtIO disk drivers improve the disk imaging throughput for my machine by about 15 percent. Also, don't forget to preload the VirtIO drivers on a Windows machine before imaging and restoring. Otherwise you'll get BSOD on boot.