qmp: Add runnability information to query-cpu-definitions

Add a new optional field to query-cpu-definitions schema:
"unavailable-features". It will contain a list of QOM properties
that prevent the CPU model from running in the current host.

Cc: David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: Michael Mueller <mimu@linux.vnet.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Jiri Denemark <jdenemar@redhat.com>
Cc: libvir-list@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
