display the entities that are referencing the current entity with views
You create a View that lists all reservations and then add entity_reference_field as the context.

If you use Panels, you should create a content pane and require a node id as context.

If you don't use panels, you should create a block and provide a default value to the context being the node id from the url. (When you insert the block on the node page, views will grab the node id from the url and use it in the query to display only reservations for that room).