qemu.tests.cfg: Disable virtio_scsi related cases for old rhel guest

virtio-scsi driver support from RHEL6.3, so disable old rhel guest.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #228 from spcui/multi_disk_fix

qemu.tests.cfg: Disable virtio_scsi related cases for old rhel guest