qmp: Fix device-list-properties not to crash for abstract device Broken in commit f4eb32b "qmp: show QOM properties in device-list-properties", v2.1. Cc: qemu-stable@nongnu.org Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Andreas Färber <afaerber@suse.de> Message-Id: <1443689999-12182-9-git-send-email-armbru@redhat.com>