Brings size based backup chains (--auto-full-ratio and
--auto-full-remove-chain), correct cleanup of an interrupted backup
(0.53) and of a backup job failing on a full target filesystem (0.58).

Since 0.63 the backup chain directory is named after the disk image file
instead of the qemu node name, so each existing deployment drops its
chain and bitmaps and pays one full backup. That name is also the one of
the old style backup file, which thus can no longer be deleted after the
backup, as qmpbackup would not be able to create its directory.