virtio-serial-bus: fix guest_connected init before driver init

virtio-serial-bus: fix guest_connected init before driver init

guest_connected should be false before guest driver initialization, and
true after, both for multiport aware and non multiport aware drivers.

Don't set it before the guest_features are available; instead use
set_status which is called by io to VIRTIO_PCI_STATUS with
VIRTIO_CONFIG_S_DRIVER_OK by even older non multiport drivers.

[Amit: Add comment, tweak summary, only set guest_connected and not
       reset it as a side-effect.]

Signed-off-by: Alon Levy <alevy@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>