PS:
[rocketmouse@archlinux ~]$ lsmod | grep usbcore
usbcore 245760 7
usbhid,snd_usb_audio,ehci_hcd,xhci_pci,snd_usbmidi_lib,xhci_hcd,ehci_pci
usb_common 16384 1 usbcore
The guest still needs the host's *hci* modules, but blacklisting the
snd* modules might allow the guest's software to get access to some
features. However, even if this should be the case, as you already
pointed out, this doesn't make sense, since you want to access features
by the guest, to improve working with the host, accessing the Focusrite
by the class compliant host's driver.