qemu: Enforce WWN to be unique among VM's disks

Operating systems use the identifier to name the disks. As the name
suggests the ID should be unique.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1208009

docs/formatdomain.html.in

@@ -2453,7 +2453,8 @@
       <dt><code>wwn</code></dt>
       <dd>If present, this element specifies the WWN (World Wide Name)
         of a virtual hard disk or CD-ROM drive. It must be composed
-        of 16 hexadecimal digits.
+        of 16 hexadecimal digits and must be unique (at least among
+        disks of a single domain)
         <span class='since'>Since 0.10.1</span>
       </dd>
       <dt><code>vendor</code></dt>