virt: sevguest: Add support to get extended report

Version 2 of GHCB specification defines Non-Automatic-Exit(NAE) to get
the extended guest report. It is similar to the SNP_GET_REPORT ioctl.
The main difference is related to the additional data that will be
returned. The additional data returned is a certificate blob that can
be used by the SNP guest user. The certificate blob layout is defined
in the GHCB specification. The driver simply treats the blob as a opaque
data and copies it to userspace.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>