iotests: Use configured python

iotests: Use configured python

Currently, QEMU's iotests rely on /usr/bin/env to start the correct
Python (that is, at least Python 2.4, but not 3). On systems where
Python 3 is the default, the user has no clean way of making the iotests
use the correct binary.

This commit makes the iotests use the Python selected by configure.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>