By default, libvirt will use this path for the VM images:
Code:
/var/lib/libvirt/images
so in your case, it will use the /root partition. but when you run virt-manager, you can specify a new data pool and put all your images in that location (a.k.a., directory). or you can just symlink /var/lib/libvirt/images to the directory on the partition you wish. so you could do something like make a dir in /home called "images" then symlink to that.
what I do is set up my largest partition as /data then put things like VM images in there (e.g., /data/libvirt/images).