char: use common function to disable callbacks on chardev close

This deduplicates code used a lot of times.

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 26da70c7)

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