Use the retry decorator for methods adding/removing virt res
Use the retry decorator for methods adding/removing virt res

In some cases, the add_virt_resource method fails because of what
seems to be a Hyper-V race condtion.

This change adds a retry decorator for this method, as well as the
one removing virt resources.

Change-Id: Ib526181b95ed84bdbc755e549a960ab2ca8d285c

openstack/os-win