i386/tdx: Error and exit when named cpu model is requested

i386/tdx: Error and exit when named cpu model is requested

Currently, it gets below error when requesting any named cpu model with
"-cpu" to boot a TDX VM:

  qemu-system-x86_64: KVM_TDX_INIT_VM failed: Invalid argument

It misleads people to think it's the bug of KVM or QEMU. It is just that
current QEMU doesn't support named cpu model for TDX.

To support named cpu models for TDX guest, there are opens to be
finalized and needs a mount of additional work.

For now, explicitly check the case when named cpu model is requested.
Error report a hint and exit.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250612133801.2238342-1-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>