vu_cleanup() resets most virtqueue state (started, notification,
file descriptors) but does not reset the enable flag. After a
QEMU disconnect and reconnect, the stale enable flag causes
vu_set_vring_enable_exec() to hit its early return check
(vq->enable == enable), skipping the threading_start_thread() call
and leaving the worker thread stopped.
Reset vq->enable to false in vu_cleanup() alongside the other
virtqueue state.
Signed-off-by: Laurent Vivier