我可以用qemu命令启动内核：
qemu-system-arm -M vexpress-a9 -smp 1 -m 256 -kernel zImage -dtb vexpress-v2p-ca9.dtb -drive file=rootfs.ext2,if=sd,format=raw -append "console=ttyAMA0,115200 root=/dev/mmcblk0" -serial stdio -net nic,model=lan9118 -net user
但我无法使用以下命令将usb磁盘添加到quem：
qemu-system-arm -M vexpress-a9 -smp 1 -m 256 -kernel zImage -dtb vexpress-v2p-ca9.dtb -drive file=rootfs.ext2,if=sd,format=raw -append "console=ttyAMA0,115200 root=/dev/mmcblk0" -serial stdio -net nic,model=lan9118 -net user -usb -usbdevice disk:format=raw:sd.bin
audio: Could not init `oss' audio driver
qemu-system-arm: -usbdevice disk:format=raw:sd.bin: Error: no usb bus to attach usbdevice disk, please try -machine usb=on and check that the machine model supports USB
qemu: could not add USB device 'disk:format=raw:sd.bin'
如何将USB磁盘添加到qemu vexpress-a9主板？