target/i386/sev: add support to query the attestation report

target/i386/sev: add support to query the attestation report

The SEV FW >= 0.23 added a new command that can be used to query the
attestation report containing the SHA-256 digest of the guest memory
and VMSA encrypted with the LAUNCH_UPDATE and sign it with the PEK.

Note, we already have a command (LAUNCH_MEASURE) that can be used to
query the SHA-256 digest of the guest memory encrypted through the
LAUNCH_UPDATE. The main difference between previous and this command
is that the report is signed with the PEK and unlike the LAUNCH_MEASURE
command the ATTESATION_REPORT command can be called while the guest
is running.

Add a QMP interface "query-sev-attestation-report" that can be used
to get the report encoded in base64.

Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Tom Lendacky <Thomas.Lendacky@amd.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org
Reviewed-by: James Bottomley <jejb@linux.ibm.com>
Tested-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210429170728.24322-1-brijesh.singh@amd.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>