Implement better support for simplestreams.CloudSpec.
Add comments in newRawInstance.
Clean up disk-related code (diskSpec & getDisks).
provider/gce: Address TODO comment about tags.
provider/gce: Address TODO about simplestreams.HasRegion.
provider/gce: Address TODO about copying nested pointers by value.
Drop some TODOs.
provider/gce: Add disk-related constants.
provider/gce: Set the proper config default values.
provider/gce: Set the proper virt type (kvm) for instances.
provider/gce: Add a TODO about deleting disks.