For future reference:

1. Resize base disks:
   • louvre: for i in $(seq 1 8); do qm resize $((200 + i)) scsi0 +20G
   • beaubourg: for i in $(seq 9 16); do qm resize $((200 + i)) scsi0 +20G
2. Install parted: sudo clush -w @swh-workers 'apt-get install -y parted'
3. Use parted to resize the single partition: sudo clush -w @swh-workers 'echo resizepart 1 -1s | parted /dev/sda' (-1s = up to the last sector)
4. Extend the LVM physical volume: sudo clush -w @swh-workers 'pvresize /dev/sda1'
5. Extend the LVM logical volume (and resize the partition): sudo clush -w @swh-workers 'lvresize -r -l +100%FREE $(hostname)-vg/root' (-r: resize filesystem; -l +100%FREE: add 100% of the free space to the logical volume)

Enjoy your new space.