i386/tdx: Set and check kernel_irqchip mode for TDX

i386/tdx: Set and check kernel_irqchip mode for TDX

KVM mandates kernel_irqchip to be split mode.

Set it to split mode automatically when users don't provide an explicit
value, otherwise check it to be the split mode.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-39-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>