KVM: x86: remove KVM_X86_OP_NULL and mark optional kvm_x86_ops

The original use of KVM_X86_OP_NULL, which was to mark calls
that do not follow a specific naming convention, is not in use
anymore.  Instead, let's mark calls that are optional because
they are always invoked within conditionals or with static_call_cond.
Those that are _not_, i.e. those that are defined with KVM_X86_OP,
must be defined by both vendor modules or some kind of NULL pointer
dereference is bound to happen at runtime.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>