hw/ppc/mac_newworld: do not require CONFIG_USB

hw/ppc/mac_newworld: do not require CONFIG_USB

With --without-default-devices it should not be required to have
devices in the binary that are removed by -nodefaults.  It should be
therefore possible to build a binary that does not include any USB
host controller or any of the code guarded by CONFIG_USB.  While the
simpler creation functions such as usb_create_simple can be inlined,
this is not true of usb_bus_find().  Remove it, replacing it with a
search of the single USB bus on the machine.

With this change, it is possible to change "select USB_OHCI_PCI" into
an "imply" directive.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240223124406.234509-7-pbonzini@redhat.com>
[PMD: Fixed style]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>