pseries: Consolidate construction of /vdevice device tree node

pseries: Consolidate construction of /vdevice device tree node

Construction of the /vdevice node (and its children) is divided between
spapr_create_fdt_skel() (at init time), which creates the base node, and
spapr_populate_vdevice() (at reset time) which creates the nodes for each
individual virtual device.

This consolidates both into a single function called from
spapr_build_fdt().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>