197 is one example where _make_test_img is used twice without stopping
the NBD server in between. An error will occur like this:

    @@ -26,9 +26,13 @@

     === Partial final cluster ===

    +qemu-img: TEST_DIR/t.IMGFMT: Failed to get "resize" lock
    +Is another process using the image?
     Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1024
    +Failed to find an available port: Address already in use
     read 1024/1024 bytes at offset 0

Patch _make_test_img to stop the old qemu-nbd before starting a new one,
which fixes this problem, and similarly 215.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>