KVM: x86: add config for non-kvm users of page tracking

Add a config option that allows kvm to determine whether or not there
are any external users of page tracking.

Signed-off-by: David Stevens <stevensd@chromium.org>
Message-Id: <20210922045859.2011227-2-stevensd@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>