AUTHORS: Update

Latest patchset enabling libvirt on s390(x) was developed by
  Viktor Mihajlovski   <mihajlov@linux.vnet.ibm.com>
  Thang Pham           <thang.pham@us.ibm.com>
Add them to the AUTHORS file.

AUTHORS

@@ -244,6 +244,8 @@ Patches have also been contributed by:
   Gao feng             <gaofeng@cn.fujitsu.com>
   Dipankar Sarma       <dipankar@in.ibm.com>
   Gerd Hoffmann        <kraxel@redhat.com>
+  Viktor Mihajlovski   <mihajlov@linux.vnet.ibm.com>
+  Thang Pham           <thang.pham@us.ibm.com>
 
   [....send patches to get your name here....]