qemu: command: Don't downgrade to '-usb' for ppc based machines

qemu: command: Don't downgrade to '-usb' for ppc based machines

 - '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>