add disk_id/file_id options for disk images

[oliverkurth]

By default, disks and files get their IDs automatically, using diskn or filem as their IDs, where n and m are consecutive numbers. Sometimes, any post processing expects certain IDs. This PR adds options to set custom ids for disks and files.

Example:

    rootdisk:
        type: hard_disk
        parent: sata1
        disk_image: dummy.vmdk
        disk_id: dummy
        file_id: dummy_id

disk_id and file_id are independent from each other, any one can be set, or both.