For image formats that are not "QEMU native", but supported for
compatibility, it is useful to verify that an image created with
the 'gold standard' native tool can be read / written to successfully
by QEMU.

In addition to testing non-native images, this could also be useful to
test against image files created by older versions of QEMU.

This provides a directory to store small sample images, for use by
scripts in tests/qemu-iotests.

Image files should be compressed with bzip2.

To use a sample image from a bash script, the _use_sample_img function
will copy and decompress the image into $TEST_DIR, and set $TEST_IMG to
be the decompressed sample image copy.  To cleanup, call
_cleanup_test_img as normal.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>