unattended_install.cfg: limit disk num to 6 in multi_disk_install when using ahci

An ahci controller can have up to 6 downstream devices,
so need to limit disk num to 6.

history:
  V2:
    add image1 in the images value
  V1:
    assign 6 stg name as the images value

Signed-off-by: Xiangmin Han <xhan@redhat.com>
Merge pull request #1274 from Xiangmin/limit_ahci_disks_fix

unattended_install.cfg: limit disk num to 6 in multi_disk_install when sing ahci