iotests: Fix 205 for concurrent runs

Tests should place their files into the test directory.  This includes
Unix sockets.  205 currently fails to do so, which prevents it from
being run concurrently.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20190618210238.9524-1-mreitz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/205

@@ -24,7 +24,7 @@ import iotests
 import time
 from iotests import qemu_img_create, qemu_io, filter_qemu_io, QemuIoInteractive
 
-nbd_sock = 'nbd_sock'
+nbd_sock = os.path.join(iotests.test_dir, 'nbd_sock')
 nbd_uri = 'nbd+unix:///exp?socket=' + nbd_sock
 disk = os.path.join(iotests.test_dir, 'disk')