pc: Don't make die-id mandatory unless necessary

pc: Don't make die-id mandatory unless necessary

We have this issue reported when using libvirt to hotplug CPUs:
https://bugzilla.redhat.com/show_bug.cgi?id=1741451

Basically, libvirt is not copying die-id from
query-hotpluggable-cpus, but die-id is now mandatory.

We could blame libvirt and say it is not following the documented
interface, because we have this buried in the QAPI schema
documentation:

> Note: currently there are 5 properties that could be present
> but management should be prepared to pass through other
> properties with device_add command to allow for future
> interface extension. This also requires the filed names to be kept in
> sync with the properties passed to -device/device_add.

But I don't think this would be reasonable from us.  We can just
make QEMU more flexible and let die-id to be omitted when there's
no ambiguity.  This will allow us to keep compatibility with
existing libvirt versions.

Test case included to ensure we don't break this again.

Fixes: commit 176d2cda0dee ("i386/cpu: Consolidate die-id validity in smp context")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190816170750.23910-1-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>