tools: fix handling of CPU family/model/stepping in SEV validation

tools: fix handling of CPU family/model/stepping in SEV validation

The SEV-ES boot measurement includes the initial CPU register state
(VMSA) and one of the fields includes the CPU identification. When
building a VMSA blob we get the CPU family/model/stepping from the
host capabilities, however, the VMSA must reflect the guest CPU not
host CPU. Thus using host capabilities is only when whe the guest
has the 'host-passthrough' CPU mode active. With 'host-model' it is
cannot be assumed host and guest match, because QEMU may not (yet)
have a named CPU model for a given host CPU.

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