Update qemu hook to facilitate Multicast

mdns AND IPv6 Neighbour Discovery Protocol use multicast packets
and the macvtap interface drops these packets by default.

Since there is no way to tell libvirt to make this better,
lets at least use the hook to make the macvtap interface
change such that multicast packets will pass.

With this, iPXE should work... and mdns should be testable in CI.

Change-Id: I397d211675fd7668b8fd19e989d740d6bad2c70e

devstack/files/hooks/qemu.py

@@ -17,8 +17,10 @@
 import datetime
 import os
 import re
+import subprocess
 import sys
 
+
 # This script is run as a libvirt hook.
 # More information here: https://libvirt.org/hooks.html
 
@@ -42,6 +44,18 @@ def main():
     guest_name = sys.argv[1]
     action = sys.argv[2]
 
+    if action == "started":
+        interfaces = str(
+            subprocess.check_output(
+                ['ip', 'link', 'show', 'type', 'macvtap']
+            )).split("\n")
+        for iface_line in interfaces:
+            if 'macvtap' in iface_line:
+                iface_string = iface_line.split('@')
+                ifaces = iface_string[0].split(' ')
+                subprocess.call(['ip', 'link', 'set', 'dev', ifaces[1],
+                                 'multicast', 'on', 'allmulticast', 'on'])
+
     if action != "release":
         return