docs: Improve the schema for fs device

<filesystemtgt> is redundant, as every group uses it; <address>
shouldn't be in <filesystemtgt> in case of the meaning could be
"filesystemtarget"; The elements <address>, <alias>, <target>,
... should be interleaved.

docs/schemas/domaincommon.rng

@@ -1210,7 +1210,6 @@
               </attribute>
               <empty/>
             </element>
-            <ref name="filesystemtgt"/>
           </interleave>
         </group>
         <group>
@@ -1224,7 +1223,6 @@
               </attribute>
               <empty/>
             </element>
-            <ref name="filesystemtgt"/>
           </interleave>
         </group>
         <group>
@@ -1257,7 +1255,6 @@
                 <empty/>
               </element>
             </optional>
-            <ref name="filesystemtgt"/>
           </interleave>
         </group>
         <group>
@@ -1271,42 +1268,39 @@
               </attribute>
               <empty/>
             </element>
-            <ref name="filesystemtgt"/>
           </interleave>
         </group>
       </choice>
-      <optional>
-        <attribute name="accessmode">
-        <choice>
-          <value>passthrough</value>
-          <value>mapped</value>
-          <value>squash</value>
-        </choice>
-        </attribute>
-      </optional>
-      <optional>
-        <element name='readonly'>
+      <interleave>
+        <element name="target">
+          <attribute name="dir">
+            <ref name="absDirPath"/>
+          </attribute>
           <empty/>
         </element>
-      </optional>
+        <optional>
+          <attribute name="accessmode">
+            <choice>
+              <value>passthrough</value>
+              <value>mapped</value>
+              <value>squash</value>
+            </choice>
+          </attribute>
+        </optional>
+        <optional>
+          <element name='readonly'>
+            <empty/>
+          </element>
+        </optional>
+        <optional>
+          <ref name="alias"/>
+        </optional>
+        <optional>
+          <ref name="address"/>
+        </optional>
+      </interleave>
     </element>
   </define>
-  <define name="filesystemtgt">
-    <interleave>
-      <optional>
-        <ref name="alias"/>
-      </optional>
-      <optional>
-        <ref name="address"/>
-      </optional>
-      <element name="target">
-        <attribute name="dir">
-          <ref name="absDirPath"/>
-        </attribute>
-        <empty/>
-      </element>
-    </interleave>
-  </define>
   <!--
       An interface description can either be of type bridge in which case
       it will use a bridging source, or of type ethernet which uses a device