chardev: Allow frontends to notify backends of guest open / close

chardev: Allow frontends to notify backends of guest open / close

Some frontends know when the guest has opened the "channel" and is actively
listening to it, for example virtio-serial. This patch adds 2 new qemu-chardev
functions which can be used by frontends to signal guest open / close, and
allows interested backends to listen to this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>