qmp: Fix device-list-properties not to crash for abstract device

Broken in commit f4eb32b5 "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>