Instead of storing the verified+decrypted chunks as intermediate files
on disk, pipe them straight into decompression+extraction. This saves a
lot of disk space and typing.
VMs whose backup size exceeds 100 GiB have more than a thousand chunks,
i.e. private.img.999 is succeeded by private.img.1000 and so on. Ensure
that these are all processed, and in the right numerical order.
FixesQubesOS/qubes-issues#6113
