virtio-serial: save/load: Ensure we have hot-plugged ports instantiated

If some ports that were hot-plugged on the source are not available on
the destination, fail migration instead of trying to deref a NULL
pointer.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Reported-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-serial-bus.c

@@ -451,6 +451,13 @@ static int virtio_serial_load(QEMUFile *f, void *opaque, int version_id)
 
         id = qemu_get_be32(f);
         port = find_port_by_id(s, id);
+        if (!port) {
+            /*
+             * The requested port was hot-plugged on the source but we
+             * don't have it
+             */
+            return -EINVAL;
+        }
 
         port->guest_connected = qemu_get_byte(f);
     }