qdev: give all devices a canonical path

A strong limitation of QOM right now is that unconverted ports
(e.g. all...) do not give a canonical path to devices that are
part of the board.  This in turn makes it impossible to replace
PROP_PTR with a QOM link for example.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
