Test cases for lun device with sgio element.

Disk order and type verification for multiple disks attachment.
Merge branch 'virtual-disks' into disk-options
Add cases for disk xml options.

Test disk cache option.
Test disk blockio element.
Test disk geometry element.
Test disk driver element.
Test wwn, serial, product option with ide/scsi disk.
More cases for disk options:

copy_on_read; dir type disk; file name with comma; discard.
remove the geometry option test as it fails to pass under some conditions.
Add disk bootorder and snapshot test cases.
Add disk encyption test cases.