Mitchellmitchellmitchell wrote:My intention is simply to increase the CPU power (?) of the guest.
By ...
hotplugging the CPUS?
How about shutting down the guest and making the changes? Especially when you say:
Mitchellmitchellmitchell wrote:The guest can be switched on/off
Then, just simply shutdown the guest, increase the numbers of CPUs at will, restart the guest. It's much easier to change the hardware, while the computer is not running! And even though you say that you don't know too much about hardware, that part should be more than obvious; you don't add more wheels to a car while you're moving, it has to be a
really special crafted car...
Mitchellmitchellmitchell wrote:I only have access to the host (and guest) over the command line
That's OK, that's the idea...
Mitchellmitchellmitchell wrote:I am using virtualboxmanage 4.3.40, which is (as I understand it) the last version for this old Ubuntu 14 system.
Wrong I'm afraid. Did you see the
Linux Downloads page? There's a specific Ubuntu 14.04 package there. Download it and install it so that we're on the same page; the page of "today"...

PS. I actually downgraded to 4.3.40 without even checking, I took your word for it. And I installed a 12.04 guest so that we're on the same page. I'm planning to "restoring order" and switch to 6.0.14. Do the same...