curl -i -X POST -H 'Accept: application/json' -H 'Content-type:application/json' -u user:password --url <api_url>/virtual_machines/:id/backups.json -d '{"backup":{"note":"manual BU"}}'

{
    "backup": {
        "allow_resize_without_reboot": true,
        "allowed_hot_migrate": true,
        "allowed_swap": true,
        "backup_server_id": 1,
        "backup_size": 0,
        "built": false,
        "built_at": null,
        "created_at": "2015-08-20T22:04:17+00:00",
        "data_store_type": "lvm",
        "id": 6254,
        "identifier": "x2wp1q3ztbkiwt",
        "initiated": "manual",
        "iqn": null,
        "locked": true,
        "marked_for_delete": false,
        "min_disk_size": 0,
        "min_memory_size": 384,
        "note": "my backup disk",
        "operating_system": "linux",
        "operating_system_distro": "rhel",
        "target_id": 489,
        "target_type": "VirtualMachine",
        "template_id": 128,
        "updated_at": "2015-08-20T22:04:17+00:00",
        "user_id": 337,
        "volume_id": null,
        "backup_type": "incremental",
        "disk_id": null
    }
}

 curl -i -X POST -H 'Accept: application/xml' -H 'Content-type:application/xml' -u user:password --url <api_url>/virtual_machines/:id/backups.xml -d '<backup><note>manual BU</note></backup>'