qemu-iotests: Add basic ability to use binary sample images

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: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>