Automatically skip ISO disks from backups

[rsmontero]

Description
When a VM includes a CD drive with an ISO image it should be skipped from the backup operation:

• Incremental backups can only be performed with qcow2 files so using a ISO image disables the incremental option
• The backup (now) of this ISO disks is done in qcow2 format that does not have much sense
• The restore operation can use the original ISO image (or none, as it can be attached any time after restoring it)

Code changes, probably here:

• https://github.com/OpenNebula/one/blob/master/src/tm/TransferManager.cc#L2364
• https://github.com/OpenNebula/one/blob/master/src/vm/VirtualMachineDisk.cc#L1561
• Maybe also something here https://github.com/OpenNebula/one/blob/master/src/tm_mad/lib/backup.rb#L315

Use case
Do not backup useless data (ISO files).
Use incremental backups in a VM with CD drives

Interface Changes
none

Progress Status

• Code committed
• Testing - QA
• Documentation (Release notes - resolved issues, compatibility, known issues)