qemu-iotests: add read/write from smaller backing image test

Some image formats support backing images that are smaller than the
image file.  This patch adds a test that verifies that reads and writes
beyond the end of backing image work.

Unallocated reads beyond the end of the backing file should produce
zeroes.

Writes beyond the end of the backing file should copy-on-write using
zeroes.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
