iotests: Add missing 'blkdebug::' in 040

040 tries to invoke pause_drive() on a drive that does not use blkdebug.
Good idea, but let's use blkdebug to make it actually work.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 20171109203025.27493-3-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/040

@@ -289,7 +289,7 @@ class TestSetSpeed(ImageCommitTestCase):
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 0 512', test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288', mid_img)
-        self.vm = iotests.VM().add_drive(test_img)
+        self.vm = iotests.VM().add_drive('blkdebug::' + test_img)
         self.vm.launch()
 
     def tearDown(self):