when the agent connection is lost, the input handler of the mouse
doesn't deactivate, which results in unresponsive mouse events in
VNC windows.

To fix this issue, call vdagent_disconnect() to reset the state
each time the frontend disconncect

Signed-off-by: Guoyi Tu <tugy@chinatelecom.cn>
Signed-off-by: dengpengcheng <dengpc12@chinatelecom.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <71fd5a58fd09f10cdb35f167b2edb5669300116e.1692281173.git.tugy@chinatelecom.cn>