qemu: Don't generate alias for model=none USB controllers

qemu: Don't generate alias for model=none USB controllers

That obviously doesn't make sense, since the value is used
to indicate the absence of a USB controller.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>