In order to have an easy way to add a new qdev with a specific id
carve out the needed functionality from qdev_device_add() into a new
function qdev_set_id().

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>