hw/ppc: modified the condition for usb controllers to be created for some ppc machines

hw/ppc: modified the condition for usb controllers to be created for some ppc machines

Some ppc machines create a default usb controller based on a 'machine condition'.
Until now the logic was: create the usb controller if:
 -  the usb option was supplied in cli and value is true or
 -  the usb option was absent and both set_defaults and the machine
    condition were true.

Modified the logic to:
Create the usb controller if:
 - the machine condition is true and defaults are enabled or
 - the usb option is supplied and true.

The main for this is to simplify the usb_enabled method.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>