schema: Introduce scaffolding for schema for <privateData> elements

schema: Introduce scaffolding for schema for <privateData> elements

Libvirt internally (e.g. in the status XML) stores additional data for
various objects described by the XML. The data is usually stored in
<privateData> or similar sub-elements.

This patch adds possibility for internal schema files to describe the
<privateData> elements by schema while still disallowing them for the
public schema.

This patch adds definitions for private data of <disk> and the
corresponding storage source of a disk.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>