virtio: console: fix raising SIGIO after port unplug

commit 92d3453815fbe74d539c86b60dab39ecdf01bb99 upstream.

SIGIO should be sent when a port gets unplugged.  It should only be sent
to prcesses that have the port opened, and have asked for SIGIO to be
delivered.  We were clearing out guest_connected before calling
send_sigio_to_port(), resulting in a sigio not getting sent to
processes.

Fix by setting guest_connected to false after invoking the sigio
function.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>