virt/coco/sev-guest: Check SEV_SNP attribute at probe time

virt/coco/sev-guest: Check SEV_SNP attribute at probe time

commit 7d93816303e99668d547b9e165b2dc51cea43a54 upstream.

No need to check it on every ioctl. And yes, this is a common SEV driver
but it does only SNP-specific operations currently. This can be
revisited later, when more use cases appear.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230307192449.24732-3-bp@alien8.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>