KVM: SEV: Provide vCPU-scoped accessors for detecting SEV+ guests

Provide vCPU-scoped accessors for detecting if the vCPU belongs to an SEV,
SEV-ES, or SEV-SNP VM, partly to dedup a small amount of code, but mostly
to better document which usages are "safe".  Generally speaking, using the
VM-scoped sev_guest() and friends outside of kvm->lock is unsafe, as they
can get both false positives and false negatives.

But for vCPUs, the accessors are guaranteed to provide a stable result as
KVM disallows initialization SEV+ state after vCPUs are created.  I.e.
operating on a vCPU guarantees the VM can't "become" an SEV+ VM, and that
it can't revert back to a "normal" VM.

This will also allow dropping the stubs for the VM-scoped accessors, as
it's relatively easy to eliminate usage of the accessors from common SVM
once the vCPU-scoped checks are out of the way.

No functional change intended.

Link: https://patch.msgid.link/20260310234829.2608037-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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