schema: make source optional in volume XML

We don't parse it anyway.

https://bugzilla.redhat.com/show_bug.cgi?id=893273

docs/schemas/storagevol.rng

@@ -20,7 +20,9 @@
           <text/>
         </element>
       </optional>
+      <optional>
         <ref name='source'/>
+      </optional>
       <ref name='sizing'/>
       <ref name='target'/>
       <optional>

tests/Makefile.am

@@ -78,6 +78,7 @@ EXTRA_DIST =		\
 	storagepoolxml2xmlin \
 	storagepoolxml2xmlout \
 	storagevolxml2argvdata \
+	storagevolschemadata \
 	storagevolschematest \
 	storagevolxml2xmlin \
 	storagevolxml2xmlout \

tests/storagevolschemadata/qcow2-no-source.xml

+<volume>
+  <name>OtherDemo.img</name>
+  <key>/var/lib/libvirt/images/OtherDemo.img</key>
+  <capacity unit="G">5</capacity>
+  <allocation>294912</allocation>
+  <target>
+    <path>/var/lib/libvirt/images/OtherDemo.img</path>
+    <format type='qcow2'/>
+    <permissions>
+      <mode>0644</mode>
+      <owner>0</owner>
+      <group>0</group>
+      <label>unconfined_u:object_r:virt_image_t:s0</label>
+    </permissions>
+    <encryption format='qcow'>
+      <secret type='passphrase' uuid='e78d4b51-a2af-485f-b0f5-afca709a80f4'/>
+    </encryption>
+  </target>
+  <backingStore>
+    <path>/dev/null</path>
+    <format type='raw'/>
+    <permissions>
+      <mode>0644</mode>
+      <owner>0</owner>
+      <group>0</group>
+      <label>unconfined_u:object_r:virt_image_t:s0</label>
+    </permissions>
+  </backingStore>
+</volume>

tests/storagevolschematest

@@ -4,7 +4,7 @@
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 
-DIRS="storagevolxml2xmlin storagevolxml2xmlout"
+DIRS="storagevolxml2xmlin storagevolxml2xmlout storagevolschemadata"
 SCHEMA="storagevol.rng"
 
 check_schema "$DIRS" "$SCHEMA"