It is not "obvious" to me that this question is on-topic for Server Fault.
Just because a tool's website says "enterprise grade" DOES NOT magically make all questions about that tool "on topic". Conversely just because a tool's website does not say "enterprise grade" doesn't make questions about that tool "off topic".
I judge each question based on its merits and try to enforce the site community's quality standards.
Server Fault deals with system administration. VirtualBox is generally an end user tool, and the use/support of end-user tools is not something we typically get into.
From my standpoint you are asking an end user question in the same vein as "How do I save a Word document as a fillable form template." I'm generally inclined to give questions the benefit of the doubt, but nothing in your question says "system administration" to me - it says "end user support request". (It also says "Didn't read the documentation" but that's a separate rant.)
If your question were about the administration of virtual box in a professional environment ("How do I deploy/update/patch Virtual Box on a bunch of workstations?" ; "We provide a legacy application as a VirtualBox image, how can I easily update that image on all my company's systems?", etc.) I would certainly say it's on-topic.
"How do I export / backup / restore my VM image?" does not, in my mind, fit the bill.
If enough users disagree with my logic (or you convince another moderator that I screwed up) the question can certainly be reopened - if the community thinks I got it wrong I certainly won't contest that.
I'm also open to being convinced you question is on-topic and reversing my own closure, but I remain unconvinced at present - I feel my reasons for closing this question still hold.