qom: Fix crash with qom-list and link properties

Commit 9561fda8d90e176bef598ba87c42a1bd6ad03ef7 changed the type of
'opaque' for link properties, but missed updating this call site.
Reproducer:

./x86_64-softmmu/qemu-system-x86_64 -qmp unix:./qmp.sock,server &
./scripts/qmp/qmp-shell ./qmp.sock
(QEMU) qom-list path=//machine/i440fx/pci.0/child[2]

Reported-by: Marcin Gibuła <m.gibula@beyond.pl>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: 2f8f007ce2152ac3b65f0811199662799c509225.1397155389.git.crobinso@redhat.com
Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
