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

        result = self.vm.qmp('block-stream', device='nonexistent')

        self.assert_qmp(result, 'error/class', 'DeviceNotFound')

class TestSmallerBackingFile(ImageStreamingTestCase):

    backing_len = 1 * 1024 * 1024 # MB

    image_len = 2 * backing_len

    def setUp(self):

        self.create_image(backing_img, self.backing_len)

        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img, str(self.image_len))

        self.vm = iotests.VM().add_drive(test_img)

        self.vm.launch()

    # If this hangs, then you are missing a fix to complete streaming when the

    # end of the backing file is reached.

    def test_stream(self):

        self.assert_no_active_streams()

        result = self.vm.qmp('block-stream', device='drive0')

        self.assert_qmp(result, 'return', {})

        completed = False

        while not completed:

            for event in self.vm.get_qmp_events(wait=True):

                if event['event'] == 'BLOCK_JOB_COMPLETED':

                    self.assert_qmp(event, 'data/type', 'stream')

                    self.assert_qmp(event, 'data/device', 'drive0')

                    self.assert_qmp(event, 'data/offset', self.image_len)

                    self.assert_qmp(event, 'data/len', self.image_len)

                    completed = True

        self.assert_no_active_streams()

        self.vm.shutdown()

class TestStreamStop(ImageStreamingTestCase):

    image_len = 8 * 1024 * 1024 * 1024 # GB

......

.......

----------------------------------------------------------------------

Ran 6 tests

Ran 7 tests

OK