qemu-iotests: add backing file smaller than image test case

This new test case checks that streaming completes successfully when the
backing file is smaller than the image file.

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