x86/virt/tdx: Make INTEL_TDX_HOST depend on X86_X2APIC

TDX capable platforms are locked to X2APIC mode and cannot fall back to
the legacy xAPIC mode when TDX is enabled by the BIOS.  TDX host support
requires x2APIC.  Make INTEL_TDX_HOST depend on X86_X2APIC.
Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/lkml/ba80b303-31bf-d44a-b05d-5c0f83038798@intel.com/
Link: https://lore.kernel.org/all/20231208170740.53979-3-dave.hansen%40intel.com

arch/x86/Kconfig

@@ -1970,6 +1970,7 @@ config INTEL_TDX_HOST
 	depends on CPU_SUP_INTEL
 	depends on X86_64
 	depends on KVM_INTEL
+	depends on X86_X2APIC
 	help
 	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
 	  host and certain physical attacks.  This option enables necessary TDX