tests/qemu-iotests: Explicit usage of Python3 (scripts without __main__)

Use the program search path to find the Python 3 interpreter.

Patch created mechanically by running:

  $ sed -i "s,^#\!/usr/bin/\(env\ \)\?python$,#\!/usr/bin/env python3," \
      $(git grep -lF '#!/usr/bin/env python' \
      | xargs grep -L 'if __name__.*__main__')

Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-11-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
diff --git a/tests/qemu-iotests/255 b/tests/qemu-iotests/255
index 0ba03d9..4a4818b 100755
--- a/tests/qemu-iotests/255
+++ b/tests/qemu-iotests/255
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Test commit job graph modifications while requests are active
 #