iotests: Stop NBD server in test 162 before starting the next one

iotests: Stop NBD server in test 162 before starting the next one

Test 162 recently started failing for me for no obvious reasons (I
did not spot any suspicious commits in this area), but looking in
the 162.out.bad log file, there was a suspicious message at the end:

 qemu-nbd: Cannot lock pid file: Resource temporarily unavailable

And indeed, the test starts the NBD server two times, without stopping
the first server before running the second one, so the second one can
indeed fail to lock the PID file. Thus let's make sure to stop the
first server before the test continues with the second one. With this
change, the test works fine for me again.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250225070650.387638-1-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>