configure: Insist on a Python 2, not Python 3

Our Python scripts require Python 2 and will fail on Python 3, eg:
  File "/home/petmay01/linaro/qemu-from-laptop/qemu/scripts/qapi-commands.py", line 378
      except getopt.GetoptError, err:
                               ^
  SyntaxError: invalid syntax

Add a check to configure that Python is not a Python 3, so we can
fail with a comprehensible error rather than an obscure one.

Reported-by: Boris Matti <swiftos@lavabit.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
