[Borgbackup] No de-duplication on big VM raw-backups/images

[Marian Beermann]

With ~2 MiB chunks, changing one bit every few MiB is sufficient to
require complete retransmission and redundant storage of the image.

Reduce chunk size or don't send a disk image but rather core data. E.g.
instead of an image of your database server, send a database dump. These
still tend to suffer from a similar issue, so instead create a classic
database backup and add differentials, clustering changes together,
further reducing transmission needs.

-Marian