It stores basic backup information like used hmac/crypto algorithm,
whether backup is encrypted/compressed and possibly more. The header
file is parsed only after successful HMAC verification. Because we do
not know which HMAC algorithm was used before reading the header, try to
guess trying all supported (starting with the default one).
Backup header is stored as the first file, which is always not encrypted
and not compressed. Then qubes.xml follows.
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