Fix detection of TDX confidential VM on Azure platform

Fix detection of TDX confidential VM on Azure platform

The original CVM detection logic for TDX assumes that the guest can see
the standard TDX CPUID leaf. This was true in Azure when this code was
originally written, however, current Azure now blocks that leaf in the
paravisor. Instead it is required to use the same Azure specific CPUID
leaf that is used for SEV-SNP detection, which reports the VM isolation
type.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>