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: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

tests/qemu-iotests/153

@@ -35,6 +35,7 @@ _cleanup()
     rm -f "${TEST_IMG}.convert"
     rm -f "${TEST_IMG}.a"
     rm -f "${TEST_IMG}.b"
+    rm -f "${TEST_IMG}.c"
     rm -f "${TEST_IMG}.lnk"
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15