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>
As you said this only influence the virtio_scsi device. So should we not make this effect in other configurations?