I think this messages shouldn't be shown when user has already installed the suggested packages:
Starting with version 4.0.0, virtualbox has USB-1 support.
For USB-2 support, PXE-boot ability and VRDP support please emerge
Starting with version 5.0.0, virtualbox no longer has the "additions" and
the "extension" USE flag. For installation of the guest additions ISO
image, please emerge
and for the USB2, USB3, VRDP and PXE boot ROM modules, please emerge