RNG schema: allow plain @floor to <bandwidth/>

The <inbound/> element to <bandwidth/> has several attributes from
which two are mandatory. Well, from two at least one has to be
present: @average or @floor or both. Instead of inventing crazy RNG
schema, let's make all the attributes optional there and rely on our
parsing code to correctly handle the situation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/schemas/networkcommon.rng

@@ -152,9 +152,11 @@
   </define>
 
   <define name="bandwidth-attributes">
-    <attribute name="average">
-      <ref name="speed"/>
-    </attribute>
+    <optional>
+      <attribute name="average">
+        <ref name="speed"/>
+      </attribute>
+    </optional>
     <optional>
       <attribute name="peak">
         <ref name="speed"/>