schemas: rng: Add encryption and slices elements to domainsnapshot schema

Both <encryption> and <diskSourceCommon> were absent from the <source>
element defined in domainsnapshot.rng
Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

docs/schemas/domainsnapshot.rng

@@ -160,6 +160,12 @@
                     <zeroOrMore>
                       <ref name='devSeclabel'/>
                     </zeroOrMore>
+                    <optional>
+                      <ref name='diskSourceCommon'/>
+                    </optional>
+                    <optional>
+                      <ref name="encryption"/>
+                    </optional>
                     <empty/>
                   </element>
                 </optional>
@@ -179,6 +185,12 @@
                     <zeroOrMore>
                       <ref name='devSeclabel'/>
                     </zeroOrMore>
+                    <optional>
+                      <ref name='diskSourceCommon'/>
+                    </optional>
+                    <optional>
+                      <ref name="encryption"/>
+                    </optional>
                     <empty/>
                   </element>
                 </optional>