qemu-iotests/153: Fix leaked scratch images

qemu-iotests 153 left t.qcow2.c behind in the scratch directory. Make
sure to clean it up after completing the tests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
