ui/vdagent: send caps on fe_open

ui/vdagent: send caps on fe_open

The spice-vdagentd doesn't send capabilities again on host/client
disconnect (but when the session agent connects and sends a
GUEST_XORG_RESOLUTION message)

When the dbus client disconnects, vdagent_disconnect() is called to
reset the agent state. Capabilities must be negotiated again on
reconnection.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240717171541.201525-5-marcandre.lureau@redhat.com>