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>