virt: sevguest: Prep for kernel internal get_ext_report()

In preparation for using the configs-tsm facility to convey attestation
blobs to userspace, switch to using the 'sockptr' api for copying
payloads to provided buffers where 'sockptr' handles user vs kernel
buffers.

While configfs-tsm is meant to replace existing confidential computing
ioctl() implementations for attestation report retrieval the old ioctl()
path needs to stick around for a deprecation period.

No behavior change intended.

Cc: Borislav Petkov <bp@alien8.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Dionna Glaze <dionnaglaze@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Tested-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>