Include the EBS volumes in the VirtualMachine object.

When AWS returns information about a VM instance, it includes the ids of the volumes attached to the instance if the instance is using EBS. It would be nice if the VirtualMachine class provided that information; having it there is a lot easier than having to query all of the volumes and then scan the volumes to determine which ones are attached to the instance in question.

This is part 2 of the patch for dasein-cloud-aws issue #87.