Expand the image format docs to cover the new options for
the qcow, qcow2 and luks disk image formats

Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-21-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>