Allow root directory in filesystem source dir schema

Use absDirPath instead of absFilePath.

https://bugzilla.redhat.com/show_bug.cgi?id=1028107

docs/schemas/domaincommon.rng

@@ -1679,7 +1679,7 @@
           <interleave>
             <element name="source">
               <attribute name="dir">
-                <ref name="absFilePath"/>
+                <ref name="absDirPath"/>
               </attribute>
               <empty/>
             </element>
@@ -1697,7 +1697,7 @@
           <interleave>
             <element name="source">
               <attribute name="dir">
-                <ref name="absFilePath"/>
+                <ref name="absDirPath"/>
               </attribute>
               <empty/>
             </element>