Use AR relations to save fields/inputs and values

Instead of saving/destroying fields/inputs and values one at time
Use the Active Record relations to save these with a single save
call on the parent. This is done by adding the :autosave to the has_many
relations

It also includes the removal of to_i calls on the id fields. This will
help us when we switch to GIT and have strings as id's and the to_i
calls would fail
Can this just all be instantiated at once?

`MiqAeValue.new(:field_id => kids.id, instance_id => inst)`

Incidentally, why is kids plural, but is used like a singular object?