Fixing aws_instance schema

Some instance types have a block device by default. So when selecting
such an instance type, you will not set a config for the block device,
but the update/refresh func will notice one and update the state
nonetheless.

So in those cases the `block_device` becomes a `computed` field.

builtin/providers/aws/resource_aws_instance.go

@@ -137,6 +137,7 @@ func resourceAwsInstance() *schema.Resource {
 			"block_device": &schema.Schema{
 				Type:     schema.TypeSet,
 				Optional: true,
+				Computed: true,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"device_name": &schema.Schema{