sev: add sev-inject-launch-secret

AMD SEV allows a guest owner to inject a secret blob
into the memory of a virtual machine. The secret is
encrypted with the SEV Transport Encryption Key and
integrity is guaranteed with the Transport Integrity
Key. Although QEMU facilitates the injection of the
launch secret, it cannot access the secret.

Signed-off-by: Tobin Feldman-Fitzthum <tobin@linux.ibm.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Message-Id: <20201027170303.47550-1-tobin@linux.ibm.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>