build: fix text regression

Commit 8a09ee410 tickles a bug in libxml2-2.7.6 on RHEL 6.2,
where libxml2 treats the pattern [^\n] as excluding literal
backslash and n, instead of the intended newline, thus failing
to validate any domain name containing 'n'.

* docs/schemas/domaincommon.rng: Use literal newline instead.

docs/schemas/domaincommon.rng

@@ -3058,7 +3058,9 @@
   </define>
   <define name="domainName">
     <data type="string">
-      <param name="pattern">[^\n]+</param>
+      <!-- Use literal newline instead of \n for bug in libxml2 2.7.6 -->
+      <param name="pattern">[^
+]+</param>
     </data>
   </define>
   <define name="diskSerial">