Do not format CPU features without a model

Do not format CPU features without a model

For host-passthrough CPU we don't honor the CPU
features specified in the XML, but we allow
outputting them via the UPDATE_CPU flag for dumpxml,
this gives user a rough idea of what features the CPU
might have.

After restoring a managedsave'd domain, the features
might end up in the live status XML (in /var/run) without
the model. This XML cannot be parsed by the daemon after
restart and the domain might disappear.

This fix skips formatting the features for HOST_PASSTHROUGH
when UPDATE_CPU is not specified, so the newly restored domains
and newly created snapshots won't be affected.

Note: this doesn't fix existing snapshots or already restored
running domains.

https://bugzilla.redhat.com/show_bug.cgi?id=1030793
https://bugzilla.redhat.com/show_bug.cgi?id=1151885