Can I, for example, call on @Zanna to have a look at the new user's comment on my answer?
As pointed out by Mark Kirby, you can't effectively call on a particular user from a post they haven't yet commented on or edited, because an @user will not send them a notification. See this Meta Stack Exchange answer for details of when comment notifications are triggered.
If you want to seek help from high rep users*, you can always find a few lurking in Ask Ubuntu General Room (which I know you have visited before :) ). Someone will usually be willing to take a look at any problem...
If you're looking for a particular user, then, well, it depends on the user and whether they mind. I think most people won't mind being pinged in chat just to take a look at a few comments. Certainly you are (and anyone else is) welcome to ping me any time to ask my advice (I also hang out in the Downboat which is for moderation issues and the Island which is for anything at all) but there are plenty of topics I know nothing about and loads of regular users much more knowledgeable than me. I know a fair bit about how Ask Ubuntu works, not so much about other stuff (otherwise I would answer a lot more questions :/ ).
* Note that high reputation often indicates that a user has spent a lot of time here, not necessarily that they know a lot. If you take a look at people's badges and the topics they answer in, and you will get an idea of their areas of knowledge.
As regards the particular case in point, as far as I understand it, there's no way an installation in a virtual machine can see or write to any other space than that which has been allocated to it on the host. However, I've never installed Ubuntu or any other OS in a virtual machine so I don't have any experience to help the OP of that question, sorry :(