Handle writable_storages in miq_request_workflow better

The MiqPreloader.preload call was added prior to the usage of
`.writable_storages` for the miq_request_workflow.rb.  When
writable_storages was called, it would make a new call to the database
every time for each of the storages, which was not ideal.

This change makes it so that the writable_storages method will first
check to see if the proper data is already loaded, and if so, just use
that.  Otherwise, it will do the query as normal.

The other change was to change the MiqPreloader.preload to include the
`host_storages`, which properly applies the loaded records needed for
the changes to the `host` model to work.

app/models/host.rb

@@ -1750,11 +1750,19 @@ def openstack_host? # TODO: This doesn't belong here
   end
 
   def writable_storages
-    storages.where(:host_storages => {:read_only => [false, nil]})
+    if host_storages.loaded? && host_storages.all? { |hs| hs.association(:storage).loaded? }
+      host_storages.reject(&:read_only).map(&:storage)
+    else
+      storages.where(:host_storages => {:read_only => [false, nil]})
+    end
   end
 
   def read_only_storages
-    storages.where(:host_storages => {:read_only => true})
+    if host_storages.loaded? && host_storages.all? { |hs| hs.association(:storage).loaded? }
+      host_storages.select(&:read_only).map(&:storage)
+    else
+      storages.where(:host_storages => {:read_only => true})
+    end
   end
 
   def archived?

app/models/miq_request_workflow.rb

@@ -1050,7 +1050,7 @@ def allowed_storages(_options = {})
 
     rails_logger('allowed_storages', 0)
     st = Time.now
-    MiqPreloader.preload(hosts, :storages)
+    MiqPreloader.preload(hosts, :storages => {}, :host_storages => :storage)
 
     storages = hosts.each_with_object({}) do |host, hash|
       host.writable_storages.each { |s| hash[s.id] = s }