Add missing NEWS entries from stable-2.8

Some fixes where pushed to the stable-2.8 branch without a corresponding NEWS
entry. This patch adds them.
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

NEWS

@@ -10,6 +10,10 @@ Version 2.8.4
 - Change the list separator for the usb_devices parameter from comma to space.
   Commas could not work because they are already the hypervisor option
   separator (Issue 649)
+- Add support for blktap2 file-driver (Issue 638)
+- Add network tag definitions to the haskell codebase (Issue 641)
+- Fix RAPI network tag handling
+- Add the network tags to the tags searched by gnt-cluster search-tags
 
 
 Version 2.8.3