docs: Document the new <slices> sub-element of disk's <source>

docs: Document the new <slices> sub-element of disk's <source>

We are going to add support for specifying offset and size attributes
which will allow controling where the image and where the guest data
itself starts in the source of the disk. This will be represented by
a <slices> element filled with either a <slice type='storage'> for the
offset of the image format data.

Add the XML documentation and RNG schema.

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