usb-storage takes care to fetch the USB serial number from -drive
options, but it neglected to pass its own 'serial' property to the
scsi-disk it creates. With this patch, the 'serial' qdev property and
the 'serial' option in -drive behave the same and correctly apply the
serial number on both USB and SCSI level.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>