hw/mips/loongson3_virt: do not require CONFIG_USB

hw/mips/loongson3_virt: do not require CONFIG_USB

Once the Kconfig for hw/mips is cleaned up, it will be 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 created by loongson3_virt_devices_init().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20240223124406.234509-5-pbonzini@redhat.com>
[PMD: Fixed style]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>