x86/virt/tdx: Disable TDX host support when kexec is enabled

TDX host support currently lacks the ability to handle kexec.  Disable TDX
when kexec is enabled.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20231208170740.53979-20-dave.hansen%40intel.com