virtualbox.org

Guests running in IO APIC multi-core mode having a poor performance. I'd like to disable IO APIC multi-core and instead specify the host core for the guest so I can spread all guests over the host cores to balance the load.

Is it possible to choose the host core? If not, is it worth a feature request?

Such feature does not exist on Virtualbox, no.
As for the feature request, it has already been request AFAIK but will always be a very low prioirty feature for the devs as such afinity is for a very (very) limited subset of users, and such afinity will most likely cause more harm than good.
The moto is that the host OS thread scheduler should be left alone so it can do its job properly.