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

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 <[email protected]>
Suggested-by: Daniel P. Berrangé <[email protected]>
Suggested-by: Stefan Hajnoczi <[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Message-Id: <20200130163232[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>