Load Lans effeciently in MiqProvisionVirtWorkflow

Instead of preloading Lan records, splitting them up based on if they are
shareable and instantiating entire ActiveRecord to only use the name
column in a hash, use a specific query to gather and filter those values
and generate a hash from that.

app/models/miq_provision_virt_workflow.rb

@@ -230,8 +230,6 @@ def available_vlans_and_hosts(options = {})
     hosts = get_selected_hosts(src)
     unless @vlan_options[:vlans] == false
       rails_logger('allowed_vlans', 0)
-      # TODO: Use Active Record to preload this data?
-      MiqPreloader.preload(hosts, :lans => :switches)
       load_allowed_vlans(hosts, vlans)
       rails_logger('allowed_vlans', 1)
     end
@@ -244,9 +242,11 @@ def load_allowed_vlans(hosts, vlans)
   end
 
   def load_hosts_vlans(hosts, vlans)
-    hosts.each do |h|
-      h.lans.each { |l| vlans[l.name] = l.name unless l.switch.shared? }
-    end
+    Lan.joins(:switch => :hosts)
+       .where(:hosts => {:id => hosts.map(&:id)})
+       .where(:switches => {:shared => [nil, false]})
+       .distinct.pluck(:name)
+       .each_with_object(vlans) { |v, h| h[v] = v }
   end
 
   def filter_by_tags(target, options)