qemu-iotests: add qed.py image manipulation utility

The qed.py utility can inspect and manipulate QED image files.  It can
be used for testing to see the state of image metadata and also to
inject corruptions into the image file.  It also has a scrubbing feature
to copy just the metadata out of an image file, allowing users to share
broken image files without revealing data in bug reports.

This has lived in my local repo for a long time but could be useful
to others.

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