512069 fix domain XML schemas for backward compatibility

For backward compatibility we used to add the tty path as
a tty attribute on console of type pty, duplicating the value
now found in source/@path, but the Relax-NG grammar wasn't
extended for this
    <console type='pty' tty='/dev/pts/8'>
      <source path='/dev/pts/8'/>
      <target port='0'/>
    </console>

* docs/schemas/domain.rng: allow an optional tty attribute
  containing a devicePath

docs/schemas/domain.rng

@@ -931,6 +931,11 @@
     -->
   <define name="qemucdev">
     <ref name="qemucdevSrcType"/>
+    <optional>
+      <attribute name="tty">
+        <ref name="devicePath"/>
+      </attribute>
+    </optional>
     <interleave>
       <ref name="qemucdevSrcDef"/>
       <optional>