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