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>
Merge pull request #126 from spcui/multi_disk_fix

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