I installed KVM without LVM to keep it simple. My guest file server ran out of root partition space when I installed Plex Media Server. This guide helped me increase the size of my virtual disk.

SSH to kvm:

$ sudo virsh domblklist file2

fdisk -l /var/lib/libvirt/images/file2.qcow2

sudo qemu-img resize /var/lib/libvirt/images/file2.qcow2 +3G

SSH to file2:

lsblk (confirm new disk size)

sudo growpart /dev/vda 2

lsblk

sudo resize2fs /dev/vda2

df -h