- 'pseries' machines already don't allow downgrade
- 'g3beige' and 'mac99' machines use 'pci-ohci' controller with '-usb'
- all other machines ignore '-usb'
For 'g3beige' and 'mac99' libvirt already has 'pci-ohci' as contoller it
would select as one of the options when picking a model, thus it's
impossible to reach situation when '-usb' would be honoured.
All other machine types ignore it.
We can thus remove the fallback for all ppc-based machines.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
