qbus: Make child devices links

Make qbus children show up as link<> properties.  There is no stable
addressing for qbus children so we use an unstable naming convention.

This is okay in QOM though because the composition name is expected to
be what's stable.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>