accel/accel: Make TYPE_ACCEL abstract

There is no generic acceleration, we have to use specific
implementations. Make the base class abstract.

Fixes: b14a0b7469 ("accel: Use QOM classes for accel types")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200129212345.20547-3-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

accel/accel-target.c

@@ -38,6 +38,7 @@ static const TypeInfo accel_type = {
     .parent = TYPE_OBJECT,
     .class_size = sizeof(AccelClass),
     .instance_size = sizeof(AccelState),
+    .abstract = true,
 };
 
 /* Lookup AccelClass from opt_name. Returns NULL if not found */