Avoid N+1 in get_selected_hosts

`.load_ar_obj` will defer to `.load_ar_objs` when it is passed an array,
which will do a `.collect` call to `load_ar_obj` in return.  This is a
typical N+1, so this change takes the constantize code from
`.load_ar_obj` and applies it to the `allowed_hosts` to make a single
SQL call to for the needed hosts.

app/models/miq_provision_virt_workflow.rb

@@ -1077,7 +1077,9 @@ def get_selected_hosts(src)
             elsif src[:host_id]
               selected_host(src)
             else
-              load_ar_obj(allowed_hosts)
+              allowed_hosts.group_by(&:evm_object_class).flat_map do |type, objs|
+                type.to_s.camelize.constantize.where(:id => objs.map(&:id)).to_a
+              end
             end
     Rbac.filtered(hosts, :class => Host, :user => @requester)
   end