KVM: SVM: Add a comment to clarify what vcpu_svm.vmcb points at

Add a comment above the declaration of vcpu_svm.vmcb to call out that it
is simply a shorthand for current_vmcb->ptr.  The myriad accesses to
svm->vmcb are quite confusing without this crucial detail.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406171811.4043363-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>