Improve backup restore and verification to check whether VM data is missing

[andrewdavidwong]

In #3467 (comment), @marmarek wrote:

BTW currently verify-only (and actual restore) do not notice if some VM data is completely missing in the archive. Only corrupted data is detected.

[tasket]

@marmarek - I'd like to know if you consider this a dom0-only issue?

Currently restore will cycle through all VMs contained in an archive by default, and I think that if any chunk from a VM is missing then the encryption and/or compression layers will report an error. Even the inner tar command should report an error on its own if something is structurally missing (although its other layers that validate data).

dom0 is currently the only VM where the inner tar isn't checked during --verify-only. On R4.0 it still has to successfully decrypt, however.