qemu: Check if INTEL Trust Domain Extention support is enabled

qemu: Check if INTEL Trust Domain Extention support is enabled

Implement TDX check in order to generate domain feature capability
correctly in case the availability of the feature changed.

For INTEL TDX the verification is:
 - checking if "/sys/module/kvm_intel/parameters/tdx" contains the
   value 'Y': meaning TDX is enabled in the host kernel.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>