qemu-iotests: Test case for backing file deletion

Add a test case for trying to open an image file where it is impossible
to open its backing file (in this case, because it was deleted). When
doing this, qemu (or qemu-io in this case) should not crash but rather
print an appropriate error message.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
