snp-guest-req-v1b

KVM: SEV: Add certificate support for SNP_EXTENDED_GUEST_REQUEST events

Currently KVM implements a stub version of SNP Extended Guest Requests
that always supplies NULL certificate data alongside the attestation
report. Make use of the newly-defined KVM_EXIT_COCO_REQ_CERTS event to
provide a way for userspace to optionally supply this certificate data.

This implements the actual handling for KVM_EXIT_COCO_REQ_CERTS, so
allow it to be enabled via KVM_CAP_EXIT_COCO.

Signed-off-by: Michael Roth <michael.roth@amd.com>