virt: Add SEV-SNP guest driver

The SEV-SNP specification provides the guest a mechanism to communicate
with the PSP without risk from a malicious hypervisor who wishes to
read, alter, drop or replay the messages sent. The driver uses
snp_issue_guest_request() to issue GHCB SNP_GUEST_REQUEST or
SNP_EXT_GUEST_REQUEST NAE events to submit the request to PSP.

The PSP requires that all communication should be encrypted using key
specified through a struct snp_guest_platform_data descriptor.

Userspace can use SNP_GET_REPORT ioctl() to query the guest attestation
report.

See SEV-SNP spec section Guest Messages for more details.

  [ bp: Remove the "what" from the commit message, massage. ]

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-44-brijesh.singh@amd.com