vhost-user: save features if the char dev is closed

vhost-user: save features if the char dev is closed

That way the state can be correctly restored when the device is opened
again. This might happen if the backend is restarted.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1738768
Reported-by: Pei Zhang <pezhang@redhat.com>
Fixes: 6ab79a20af3a ("do not call vhost_net_cleanup() on running net from char user event")
Cc: ddstreet@canonical.com
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Message-Id: <20190924162044.11414-1-amorenoz@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>