When I start a virtual machine (like VMware or VirtualBox) and set it full screen then, from the host OS (in this case Ubuntu), I can beautifully switch to it with the Workspace Switcher. So I switch to the VM like I switch to a virtual screen. But switching back -- from the VM to the host's virtual screens -- seems to be impossible because by entering the VM I loose the host's workspace switcher.

Is there a nifty workspace switcher program that runs inside the VM and is able to switch workspaces of the host machine?

Edit in light of Frank Thomas' answer, can we configure VirtualBox (or VMware) to not send certain key combinations to the VM, but keep them to the host? Like Super+S. In that approach I would sadly have to miss the nice workspace switcher icon in the guest OS, but that's OK if at least the keyboard trick would work.