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: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/formatdomain.html.in

@@ -2878,6 +2878,9 @@
   <disk type='block' device='lun'>
     <driver name='qemu' type='raw'/>
     <source dev='/dev/sda'>
+      <slices>
+        <slice type='storage' offset='12345' size='123'/>
+      </slices>
       <reservations managed='no'>
         <source type='unix' path='/path/to/qemu-pr-helper' mode='client'/>
       </reservations>
@@ -3360,6 +3363,16 @@
             controller.
             <span class="since">Since 6.0.0</span>
           </dd>
+          <dt><code>slices</code></dt>
+          <dd>The <code>slices</code> element using its <code>slice</code>
+            sub-elements allows configuring offset and size of either the
+            location of the image format (<code>slice type='storage'</code>)
+            inside the storage source or the guest data inside the image format
+            container (future expansion).
+
+            The <code>offset</code> and <code>size</code> values are in bytes.
+            <span class="since">Since 6.1.0</span>
+          </dd>
         </dl>
 
         <p>

docs/schemas/domaincommon.rng

@@ -1596,12 +1596,31 @@
     </optional>
   </define>
 
+  <define name="diskSourceSlice">
+    <attribute name='offset'>
+      <ref name="positiveInteger"/>
+    </attribute>
+    <attribute name='size'>
+      <ref name="positiveInteger"/>
+    </attribute>
+  </define>
+
   <define name="diskSourceCommon">
     <optional>
       <attribute name="index">
         <ref name="positiveInteger"/>
       </attribute>
     </optional>
+    <optional>
+      <element name='slices'>
+        <element name='slice'>
+          <attribute name='type'>
+            <value>storage</value>
+          </attribute>
+          <ref name="diskSourceSlice"/>
+        </element>
+      </element>
+    </optional>
   </define>
 
   <define name="diskSource">