qemu: always report TDX feature caps on x86

qemu: always report TDX feature caps on x86

Currently domain capabilities will only ever report

    <tdx supported='yes'/>

so it is not possible to determine whether libvirt itself is
new enough to have TDX support or not, vs the host OS lacking
it.

For SEV and s390 prot-virt, the capability is always reported
whether supported or not, so do likewise for TDX, so other
x86 hosts get:

    <tdx supported='no'/>

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>