usb: fail usbdevice_create() when there is no USB bus

usb: fail usbdevice_create() when there is no USB bus

Report an error instead of segfaulting when attaching a USB device to a
machine with no USB busses:

  $ qemu-system-arm -machine vexpress-a9 \
      -sd Fedora-17-armhfp-vexpress-mmcblk0.img \
      -kernel vmlinuz-3.4.2-3.fc17.armv7hl \
      -initrd initramfs-3.4.2-3.fc17.armv7hl.img \
      -usbdevice disk:format=raw:test.img

Note that the vexpress-a9 machine does not have a USB host controller.

Reported-by: David Abdurachmanov <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>