Key rotation

As in the Intel SGX architecture, the memory encryption keys and the KMS sealing key should be rotated. For the KMS, unlike SGX, this can be done without shutting down the KMS. Rotating the memory encryption keys could be easily done by migrating the VM, which is a feature of AMD SEV-SNP. We allow the cloud provider's attack vector to never migrate the VM for simplicity. Also, this is not too much of a threat because the keys are hardware-secured and guessing an AES-128 key is not possible with current technology in the foreseeable future.