KVM: x86: allow defining return-0 static calls

A few vendor callbacks are only used by VMX, but they return an integer
or bool value.  Introduce KVM_X86_OP_OPTIONAL_RET0 for them: if a func is
NULL in struct kvm_x86_ops, it will be changed to __static_call_return0
when updating static calls.

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