The default USB device auto-selection code for 'pseries' machines picks
controller models which are also selected when '-usb' is used thus it's
impossible to end up in the case when using '-usb' would be possible:
$ qemu-system-ppc64 --machine pseries,usb=on
qemu-system-ppc64: could not find a module for type 'nec-usb-xhci'
$ qemu-system-ppc64 --machine pseries-2.5,usb=on
qemu-system-ppc64: could not find a module for type 'pci-ohci'
Remove the impossible downgrade and adjust tests.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
