Don't verify CPU features with host-passthrough

Don't verify CPU features with host-passthrough

Commit fba6bc4 introduced the non-migratable invtsc feature,
breaking save/migration with host-model and host-passthrough.

On hosts with this feature present it was automatically included
in the CPU definition, regardless of QEMU support.

Commit de0aeaf stopped including it by default for host-model,
but failed to fix host-passthrough.

This commit ignores checking of CPU features with host-passthrough,
since we don't pass them to QEMU (only -cpu host is passed),
allowing domains using host-passthrough that were saved with
the broken version of libvirtd to be restored.

https://bugzilla.redhat.com/show_bug.cgi?id=1147584