Fix size probing for VDI images
Fix size probing for VDI images

Commit 027bf2ea used the wrong offset: the text field at the start
of the header has 64 bytes, not 68. [1]

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=921452

[1] https://forums.virtualbox.org/viewtopic.php?p=29267#p29267

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