Make product entity work with devel generate

As we approach beta, I find this increasingly important - it's hard to develop and test some things (e.g. catalog, reports, etc.) without content present, and it's a pain to manually generate enough content to be useful for testing these things.

When running Devel generate, I get these fatal errors:

Drupal\Core\Database\IntegrityConstraintViolationException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'model' cannot be null: INSERT INTO {uc_products} (vid, nid, model, cost, price, weight, weight_units, length, width, height, length_units, pkg_qty, default_qty, shippable) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13); Array ( [:db_insert_placeholder_0] => 3 [:db_insert_placeholder_1] => 3 [:db_insert_placeholder_2] => [:db_insert_placeholder_3] => 0.00 [:db_insert_placeholder_4] => 0.00 [:db_insert_placeholder_5] => [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => [:db_insert_placeholder_8] => [:db_insert_placeholder_9] => [:db_insert_placeholder_10] => [:db_insert_placeholder_11] => 1 [:db_insert_placeholder_12] => 1 [:db_insert_placeholder_13] => 1 ) in Drupal\Core\Database\Connection->handleQueryException() (line 673 of /home/d0rha/www/core/lib/Drupal/Core/Database/Connection.php).

Drupal\Core\Entity\EntityStorageException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'model' cannot be null: INSERT INTO {uc_products} (vid, nid, model, cost, price, weight, weight_units, length, width, height, length_units, pkg_qty, default_qty, shippable) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13); Array ( [:db_insert_placeholder_0] => 3 [:db_insert_placeholder_1] => 3 [:db_insert_placeholder_2] => [:db_insert_placeholder_3] => 0.00 [:db_insert_placeholder_4] => 0.00 [:db_insert_placeholder_5] => [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => [:db_insert_placeholder_8] => [:db_insert_placeholder_9] => [:db_insert_placeholder_10] => [:db_insert_placeholder_11] => 1 [:db_insert_placeholder_12] => 1 [:db_insert_placeholder_13] => 1 ) in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 757 of /home/d0rha/www/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).

Perhaps related to #2355731: EntityFieldQuery on uc_product node types not working.

It's unclear to me what exactly Devel requires from a content type in order to be able to generate nodes of that type.