A client may be interested in getting the list of both abstract and
non-abstract types.  Instead of requiring them to make multiple queries
with different filter arguments, just return an 'abstract' field in
'qom-list-types'.

In addition to the new test code for validating this field, update the
abstract-interfaces test case to query for all 'interface' subtypes
(including abstract ones), and to look at the 'abstract' field directly.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170707122215.8819-3-ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>