char: move backends' io watch tag to CharDriverState

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 7ba9addc)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>