Make qemu hook running with python3

This switches the piece of code turns on multicast [1] to run with
python3.

[1] https://review.opendev.org/#/c/665221

Change-Id: I12d3764d087fa19aca5b285cbd63a19b1770bb41

devstack/files/hooks/qemu.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -tt
+#!/usr/bin/python3
 
 # Copyright (c) 2017 Intel Corporation
 #
@@ -45,10 +45,9 @@ def main():
     action = sys.argv[2]
 
     if action == "started":
-        interfaces = str(
-            subprocess.check_output(
-                ['ip', 'link', 'show', 'type', 'macvtap']
-            )).split("\n")
+        interfaces = subprocess.check_output(
+            ['ip', 'link', 'show', 'type', 'macvtap']
+        ).decode().split("\n")
         for iface_line in interfaces:
             if 'macvtap' in iface_line:
                 iface_string = iface_line.split('@')