qemu-iotests: Fix blkdebug in VM drive in 030

The test test_stream_pause in this class uses vm.pause_drive, which
requires a blkdebug driver on top of image, otherwise it's no-op and the
test running is undeterministic.

So add it.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/030

@@ -35,7 +35,7 @@ class TestSingleDrive(iotests.QMPTestCase):
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
         qemu_io('-c', 'write -P 0x1 0 512', backing_img)
-        self.vm = iotests.VM().add_drive(test_img)
+        self.vm = iotests.VM().add_drive("blkdebug::" + test_img)
         self.vm.launch()
 
     def tearDown(self):