kvm: x86: SVM: use vmcb* instead of svm->vmcb where it makes sense

This makes the code a bit shorter and cleaner.

No functional change intended.
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220322172449.235575-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>