Storage: Fixed the process_button method to render the page correctly

Rendering errors that occurred after the refresh/power button tasks were addressed in the Storage area in the VM/Hosts list view.

issue #243
@h-kataria Check out line#119 below.
```
page.replace_html("main_div", :partial=>"layouts/gtl", :locals=>{:action_url=>"show/{@ems.id}"})
```
```@ems``` is being used there without being set, which was causing the rendering to blow up.

But since its not being used anywhere else in the cim_instance controller we can get rid of it and change it to a local variable.