fix GuestDevice#child_device references

The child_devices added unneeded scope.
This scope pulled back all ids from the guest devices table.

Since the foreign_key is added to the query, this did not do any harm.
But it adds an unneeded constraint an unneeded query.

This was introduce in ManageIQ#15371
The way the PR was batted around, it looked like this was not an intentional side effect.

Before
======

```ruby
has_many :child_devices, -> { where(:parent_device_id => ids) }, :foreign_key => "parent_device_id"
has_many :child_devices, -> { where(:parent_device_id => GuestDevice.ids) }, :foreign_key => "parent_device_id"

SELECT "guest_devices".*
FROM "guest_devices"
WHERE "guest_devices"."parent_device_id" = 26000000000035
  AND "guest_devices"."parent_device_id" IN (26000000000032, 26000000000033, 26000000000034, 26000000000035, 26000000000036, 26000000000037, 26000000000038)
```

After
=====

```ruby
has_many :child_devices, :foreign_key => "parent_device_id"

SELECT "guest_devices".*
FROM "guest_devices"
WHERE "guest_devices"."parent_device_id" = 26000000000035
```

app/models/guest_device.rb

@@ -14,7 +14,7 @@ class GuestDevice < ApplicationRecord
   has_many :miq_scsi_targets, :dependent => :destroy
 
   has_many :firmwares, :dependent => :destroy
-  has_many :child_devices, -> { where(:parent_device_id => ids) }, :foreign_key => "parent_device_id", :class_name => "GuestDevice", :dependent => :destroy
+  has_many :child_devices, :foreign_key => "parent_device_id", :class_name => "GuestDevice", :dependent => :destroy
 
   has_many :physical_network_ports, :dependent => :destroy
   has_many :connected_physical_switches, :through => :physical_network_ports

spec/models/guest_device_spec.rb

@@ -31,4 +31,15 @@
     expect(template_gd.host).to be_nil
     expect(host_gd.host).to eq(host)
   end
+
+  describe "#child_device" do
+    it "brings back children" do
+      parent = FactoryBot.create(:guest_device)
+      child1 = FactoryBot.create(:guest_device, :parent_device_id => parent.id)
+      child2 = FactoryBot.create(:guest_device, :parent_device_id => parent.id)
+      FactoryBot.create(:guest_device) # sad path (though the let! probably created lots of those)
+
+      expect(parent.reload.child_devices).to match_array([child1, child2])
+    end
+  end
 end