Not sure about Debian but in Ubuntu I had to do the following after setting it up in the VB GUI:
mkdir /home/username/OSX # one time
sudo mount -t vboxsf OSX /home/username/OSX # each time
Update 02/03/2020
Kinto - Type in Linux like it's a Mac.
Kinto has now been rewritten in C for Ubuntu/Debian systems using x11. It also uses json config files, making it easier to manage and extend to other applications than just terminals. The app no longer maps to Super in the Terminal apps, it will now properly map to Ctrl+Shift to create the exact same feel as having a Cmd key.
Please checkout the latest release.
https://github.com/rbreaves/kinto
The main change to allow for the Super = Ctrl+Shift change is in this symbols file.
default partial xkb_symbols "mac_levelssym" {
key <LWIN> {
repeat= no,
type= "ONE_LEVEL",
symbols[Group1]= [ Hyper_L ],
actions[group1]=[ SetMods(modifiers=Shift+Control) ]
};
key <RWIN> {
repeat= no,
type= "ONE_LEVEL",
symbols[Group1]= [ Hyper_R ],
actions[group1]=[ SetMods(modifiers=Shift+Control) ]
};
};
You may find Kinto to be useful. It's a project I recently created, after a few years of tackling this problem a few different ways and I just now feel like it's solid enough to share.
The Kinto installer simply uses python to do the initial install, after that it's just a systemd service, bash, xprop, setxkbmap and xkbcomp. As far as I am concerned it is the fastest and most native solution to this problem that you are going to find.
https://github.com/rbreaves/kinto
https://medium.com/@benreaves/kinto-a-mac-inspired-keyboard-mapping-for-linux-58f731817c0
Here's a Gist as well, if you just want to see what is at the heart of it all, it will not alternate your keymap when needed though. The Gist also does not include custom xkb keymap files that setup macOS style cursors/word-wise manipulations that use Cmd and the arrow keys.
https://gist.github.com/rbreaves/f4cf8a991eaeea893999964f5e83eebb
Best Answer
At the present time VirtualBox does not officially support nested virtualization, even thought one can nest a 32-bit Guest a 64-bit guest cannot be nested as VT-x is not passed through to the nested Guest and it's one of the requirements to run a 64-bit Guest, nested or not.
VMware Fusion does support some nested 64-bit Guests although off the top of my head I'd have to check the documentation to be more specific. You can have a look Running Nested VMs at VMware's website.