iotests.py: qemu_nbd_popen: remove pid file after use

To not interfere with other qemu_nbd_popen() calls in same test.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210205163720.887197-8-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>