backups: do not use tar multi-volume feature, backup format 3 (#902)
backups: do not use tar multi-volume feature, backup format 3 (#902)

Tar multi-volume support is broken when used with sparse files[1], so do
not use it. Instead simply cut the archive manually and concatenate at
restore time. This change require a little modification in restore
process, so make this new backup format ("3"). Also add backup format
version to the header, instead of some guessing code.
For now only cleartext and encrypted backups implemented, compression
will come as a separate commit.

woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin