Improve the algorithm that tries to guess the disk layout:
1. Use CD-ROMs to read ISO only
2. Make explicit paths for -scsi and -blk virtio

Acked-by: Maxim Samoylov <max7255@linux.vnet.ibm.com>
Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>