SmallStack: Implement manual backup/restore for VMs

[rgooch]

With vm-control a VM owner should be able to save the VM metadata and volume data to a remote storage system (i.e. AWS S3) and then later restore/rebuild the VM in case of disaster.

The Hypervisor will have to be given credentials to write to the S3 bucket. Undecided whether vm-control will provide the credentials from the user or if the Fleet Manager will provide the credentials. For automated backups (issue #469), the Fleet Manager will need credentials anyway.