hw/ppc/pseries: do not require CONFIG_USB

hw/ppc/pseries: do not require CONFIG_USB

With --without-default-devices it is possible to build a binary that
does not include any USB host controller and therefore that does not
include 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.

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: <20240223124406.234509-8-pbonzini@redhat.com>
[PMD: Fixed style]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>