Revert "iotests: Use configured python"

This reverts commit f915db07ef9c368ea6db6430256de064fdd1525f.

This commit is broken because it does not account for the
build tree and the source tree being different, and can cause
build failures for out-of-tree builds. Revert it until we can
identify a better solution to the problem.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1400153676-30180-1-git-send-email-peter.maydell@linaro.org
Acked-by: Kevin Wolf <kwolf@redhat.com>