The threaded VNC servers messed up with QEMU fd handlers without
any kind of locking, and that can cause some nasty race conditions.

Using qemu_mutex_lock_iothread() won't work because vnc_dpy_cpy(),
which will wait for the current job queue to finish, can be called with
the iothread lock held.

Instead, we now store the data in a temporary buffer, and use a bottom
half to notify the main thread that new data is available.

vnc_[un]lock_ouput() is still needed to access VncState members like
abort, csock or jobs_buffer.
Signed-off-by: NCorentin Chary <corentin.chary@gmail.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>