virtio: console: tell host of open ports after resume from s3/s4

If a port was open before going into one of the sleep states, the port
can continue normal operation after restore.  However, the host has to
be told that the guest side of the connection is open to restore
pre-suspend state.

This wasn't noticed so far due to a bug in qemu that was fixed recently
(which marked the guest-side connection as always open).

CC: stable@vger.kernel.org   # Only for 3.3

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