qom: Fix memory leak in object_property_set_link()

Save the result of the call to object_get_cannonical_path()
so we can free it.

Cc: qemu-stable@nongnu.org
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
qtest: Use -display none by default

This avoids each test needing to add it to suppress windows popping up.

Signed-off-by: Andreas Färber <afaerber@suse.de>