qemu.tests.cfg: Small bug fix when booting vm with virtio_scsi

The stg bus number will inflict with image1 as both of them are virtio_scsi
device, so change it, should be started from 1.

Signed-off-by: Shuping Cui <scui@redhat.com>
here use "+="  in cfg files. So this update will make two drive_bus:(....) for virtio_scsi. I guess this is not what we expect.