docs: Fix validation of spapr-vio addresses

According to sPAPR, addresses are 32-bit (8 hex digits) rather
than 64-bit (16 hex digits). Update the schema accordingly.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

docs/schemas/domaincommon.rng

@@ -6391,7 +6391,7 @@
   </define>
   <define name="spaprvioReg">
     <data type="string">
-      <param name="pattern">(0x)?[0-9a-fA-F]{1,16}</param>
+      <param name="pattern">(0x)?[0-9a-fA-F]{1,8}</param>
     </data>
   </define>
   <define name='aliasName'>