qemu-iotests: add iotests Python module

qemu-iotests: add iotests Python module

Block layer tests that involve QMP commands rather than qemu-img or
qemu-io are not well-suited for shell scripting.  This patch adds a
Python module which allows tests to be written in Python instead.

The basic API is:

  VM          - class for launching and interacting with a VM
  QMPTestCase - abstract base class for tests that use QMP
  qemu_img()  - wrapper function for invoking qemu-img
  qemu_io()   - wrapper function for invoking qemu-io
  imgfmt      - the image format under test (e.g. qcow2, qed)
  test_dir    - scratch directory path for temporary files
  main()      - entry point for running tests

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>