Description of problem:

We need a guide covering everything backup developer needs to peform full
and incremental backup and restore backups.

Most of the content exists in the devconf/fosdem/deep dive slides.

Some content is available in devel mailing list.

We have lot of example code spread in ovirt-engine-sdk examples and
ovirt-imageio examples. We need one document linking all examples.

We also have lot of outdated docs for image transfer API, spread in various
places in ovirt-engine-api-model. This documentation is unusable and hard
to maintain.

Topics:

- How incremental backup works

Starting and stopping backups:
- Managing backups using engine API/SDK
- Managing image transfers using engine API/SDK
- Managing checkpoints using engine API/SDK

High level python API using files:
- Using client.upload() to restore from files
- Using clinet.download() to backup to files

Lower level API supporting streaming:
- Using ImageioClient to full backup
- Using ImageioClient to incremental backup
- Using ImageioClient to restore disks

HTTP API:
- How to do full backup using the API
- How to do incremental backup using the API
- How to restore backups using the API

This should be the base for official documentation later.

Not sure what is the right component or best place to keep this guide, but
lets create some content first and then find the best place.

Related examples:
- https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/backup_vm.py
- https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/upload_disk.py
- https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/download_disk.py
- https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/helpers/imagetransfer.py
- https://github.com/oVirt/ovirt-imageio/blob/master/examples/imageio-client
- https://github.com/oVirt/ovirt-imageio/blob/master/examples/sparse-stream