All the backends implement an io watcher tag for callbacks. Move it to
CharDriverState from each backend's struct to make accessing the tag from
backend-neutral functions easier.
This will be used later to cancel a callback on chardev detach from a
frontend.
CC: <qemu-stable@nongnu.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
(cherry picked from commit 7ba9addc16)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>