Update announce self workaround opt description

This updates the announce self workaround config opt
description to include info about instance being set
as tainted by libvirt.

Change-Id: I8140c8fe592dd54fc09a9510723892806db49a56

Author: tobias-urdin

nova/conf/workarounds.py

@@ -366,6 +366,8 @@
 the announce-self command to the QEMU monitor so that it generates RARP frames
 to update network switches in the post live migration phase on the destination.
 
+Please note that this causes the domain to be considered tainted by libvirt.
+
 Related options:
 
 * :oslo.config:option:`DEFAULT.compute_driver` (libvirt)