qmpbackup --level auto never restarts the chain, so a guest which
rewrites its disk grows the backup past the size of the disk image
itself, with 48G images seen backed up in 114G.

The exporter starts a new chain once the increments sum up to more than
backup-rolling-full-ratio times the full backup they follow, and
deletes the chain first, as the partition is not sized for the chain
and a second full backup.