KVM: SVM: Track the per-CPU host save area as a VMCB pointer

KVM: SVM: Track the per-CPU host save area as a VMCB pointer

The host save area is a VMCB, track it as such to help readers follow
along, but mostly to cleanup/simplify the retrieval of the SEV-ES host
save area.

Note, the compile-time assertion that

  offsetof(struct vmcb, save) == EXPECTED_VMCB_CONTROL_AREA_SIZE

ensures that the SEV-ES save area is indeed at offset 0x400 (whoever added
the expected/architectural VMCB offsets apparently likes decimal).

No functional change intended.

Link: https://lore.kernel.org/r/20240802204511.352017-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>