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

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

 - 'virt*' machines already don't allow downgrade
 - 'versatilepb' and 'realview' machines use 'pci-ohci' controller with '-usb'
 - all other machines ignore '-usb' (some have sysbus-based USB
   controller which we don't even consider)

For the 'versatilepb' and 'realview' machines libvirt would already
resort to picking either an existing controller model or trying to pick
the one which '-usb' would select and thus fail either way.

All other machine types ignore it.

We can thus remove the fallback for all arm-based machines.

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