schema: smbios: allow any strings

The smbios docs allow any string to be passed and libvirt does not
really do any validation on them. Allow passing any string.

Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1373535

docs/schemas/domaincommon.rng

@@ -4517,9 +4517,7 @@
   </define>
 
   <define name="sysinfo-value">
-    <data type="string">
-      <param name='pattern'>[a-zA-Z0-9/\-_\. \(\)]+</param>
-    </data>
+    <data type="string"/>
   </define>
 
   <define name="acpiTable">