tests/migration/guestperf: Use Python 3 interpreter

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200512103238.7078-7-philmd@redhat.com>

tests/migration/guestperf-batch.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 #
 # Migration test batch comparison invokation
 #

tests/migration/guestperf-plot.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 #
 # Migration test graph plotting command
 #

tests/migration/guestperf.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 #
 # Migration test direct invokation command
 #